Island Arc最新文献

{"title":"Petrogenesis of two types of gabbro from Neoproterozoic Fuchuan ophiolite complex, Jiangnan Orogen: Implication for a Japan Sea-like back-arc basin","authors":"Wentao Hong,&nbsp;Minggang Yu,&nbsp;Zhentao Song,&nbsp;Pingli Chu,&nbsp;Kai Liu,&nbsp;Xiaohua Zhou","doi":"10.1111/iar.12465","DOIUrl":"10.1111/iar.12465","url":null,"abstract":"<p>The Neoproterozoic Fuchuan ophiolite complex (FCO) represents the lithospheric remnant of the back-arc basin in the southeastern margin of the Yangtze Craton. However, the nature and development of this back-arc system are still confusing. This study focuses on the tholeiitic gabbros in the FCO, which can be divided into enriched and depleted types according to their geochemical characteristics. Enriched gabbros (843 ± 5 Ma) in the FCO are characterized by left-sloping rare earth element (REE) patterns and relatively low ε_Hf(t) (mostly in the range of 4.6–7.8). In contrast, depleted gabbros are slightly younger (838 ± 5 Ma) and isotopically more depleted (ε_Hf(t) mostly in 7.7–11.8) than enriched gabbros, exhibiting flat REE patterns. Based on geochemical variations and numerical modeling, the primary magmas of enriched gabbros were generated by a low degree (~14 %) partial melting of the melt-modified depleted mantle. In contrast, the more depleted isotopic composition and variable Ba/Th and V/Yb ratios of depleted gabbros suggest that their magmas have originated from a higher degree (~14–26 %) partial melting of the fluid-modified residual depleted mantle. Considering that the most ~860–830 Ma magmatism in the eastern Jiangnan Orogen is calc-alkaline, the Neoproterozoic back-arc basin where the FCO formed could be sialic and built on the thinning continental crust, resembling the Japan Sea. The occurrences of tholeiitic gabbros in the FCO thus provide crucial insights into the opening of the back-arc basin in the southeastern margin of the Yangtze Craton. The changes of mantle source nature and metasomatic agent reflect the upwelling of the depleted asthenosphere and oceanward retreading of the continental arc during the back-arc opening. Consequently, the early continental arc in the southeastern Yangtze Craton may have been transformed into a new island arc, resembling the formation of the northeast Honshu arc during the Miocene.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-09-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46870717","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Zircon U–Pb geochronology and geochemistry of the Late Jurassic granite porphyries from central-eastern Jilin Province, NE China: Petrogenesis and tectonic implications","authors":"Xiqing Ye,&nbsp;Zhitao Xu,&nbsp;Mengmeng Li,&nbsp;Liying Sun,&nbsp;Zhongwei Li,&nbsp;Donghan Yan,&nbsp;Jingqiao Feng","doi":"10.1111/iar.12464","DOIUrl":"10.1111/iar.12464","url":null,"abstract":"<p>The recently discovered Yizuoying Mo deposit is located in central-eastern Jilin Province, NE China. The molybdenum (Mo) mineralization, mainly hosted in granite porphyry, is considered to be granite-related. Zircon U–Pb dating of the granite porphyry yielded concordant ages of 160.81 ± 0.62 Ma, which is consistent with the weighted mean U–Pb age of 160.53 ± 0.65 Ma, indicating that the emplacement of granitic plutons occurred in the Late Jurassic. The granite porphyry samples are peraluminous, high-K calc-alkaline and show an A-type geochemical signature with high Na₂O + K₂O and Zr + Nb + Ce + Y content, K₂O/MgO, Fe₂O₃^T/(Fe₂O₃^T + MgO), REEs (rare earth elements) and 10 000 Ga/Al ratios. Based on the trace element content of zircons, they have high Th, U, Zr, Hf, and Pb abundance and obvious La and Eu anomalies, and their distribution pattern is similar to that of A-type granites. The zircon εHf(<i>t</i>) values range from 4.5 to 10.5 with Neoproterozoic <i>T</i>_DM2 ages (536–922 Ma) for Hf isotopes, and they have relatively high values of εNd(<i>t</i>) (3.14 to 3.49; <i>T</i>_DM2 = 665–693 Ma) and initial ⁸⁷Sr/⁸⁶Sr (0.723260–0.734669). Detailed elemental and isotopic data suggest that the Yizuoying granite porphyry belongs to the A₂-subtype and was formed by partial melting of a crustal source with a neoproterozoic overall residence age. Integrating new data on the oxygen fugacity of zircons and published data on the tectonic evolution, we suggest that the granite porphyry and associated Mo mineralization in the Yizuoying deposit formed in an extensional environment at ~160 Ma, related to the subduction of the Paleo-Pacific plate. The evolution of granitic magma in this period contributed to Mo mineralization.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45479695","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Solidification pressures and ages recorded in mafic microgranular enclaves and their host granite: An example of the world's youngest Kurobegawa granite","authors":"Kota Suzuki,&nbsp;Tetsuo Kawakami,&nbsp;Shigeru Sueoka,&nbsp;Ayu Yamazaki,&nbsp;Saya Kagami,&nbsp;Tatsunori Yokoyama,&nbsp;Takahiro Tagami","doi":"10.1111/iar.12462","DOIUrl":"10.1111/iar.12462","url":null,"abstract":"<p>Solidification pressures and ages of mafic microgranular enclaves (MMEs) and their host granite were determined and compared based on Al-in-hornblende geobarometry and U–Pb zircon dating in two sample localities in the Kurobegawa Granite. In sample KRG19-A03 from the middle unit of the pluton, the MME and the host granite yielded 0.18 ± 0.03 to 0.24 ± 0.04 GPa and 0.16 ± 0.03 to 0.23 ± 0.04 GPa, respectively. The MME and the host granite of sample KRG19-B08b from the lower unit, respectively, yielded 0.12 ± 0.02 to 0.21 ± 0.03 GPa and 0.13 ± 0.02 to 0.18 ± 0.03 GPa. In each sample locality, the estimated solidification pressures of the MME and its host granite overlap. The weighted mean ages were calculated as 0.775 ± 0.045 Ma and 0.831 ± 0.055 Ma for the MME and the host granite of KRG19-A03, respectively. The MME and the host granite of KRG19-B08b, respectively, yielded 0.672 ± 0.033 Ma and 0.735 ± 0.042 Ma. The ages for MMEs tend to be younger than the host granites, although they overlap within uncertainty. Zircon commonly occurs as the matrix minerals in both lithologies, meanwhile, zircon also occurs as early phases in plagioclase cores only in the host granites. Such differences in mode of occurrence of zircon suggest that the age variation reflects the differences in timing of zircon crystallization between the lithologies. Therefore, the MMEs record the same solidification pressures as the host granites and better represent the final solidification timing of the pluton. From these data of the MMEs, an average exhumation rate of each sample locality was estimated as 7.1–14.5 mm/year (KRG19-A03) and 5.5–14.4 mm/year (KRG19-B08b). These exhumation rates are much larger than that of the ca. 5.6–5.2 Ma Shiaidani Granodiorite (0.93–2.5 mm/year), implying that drastic change of the exhumation rate took place between ca. 5.2 Ma and ca. 0.83 Ma.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42082670","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Autogenic and allogenic controls on the temporal palaeographic evolution of the Himalayan foreland basin: Insights from facies analysis of the lower Siwalik succession, Kumaun Himalaya, India","authors":"Pradeep K. Goswami,&nbsp;Karuna Singh","doi":"10.1111/iar.12461","DOIUrl":"10.1111/iar.12461","url":null,"abstract":"<p>The lower Siwalik succession in the south-central Kumaun Himalaya records Middle Miocene fluvial sedimentation in the Himalayan foreland basin, the largest foreland basin of the world. Detailed facies analysis reveals three distinct facies associations, one of which is sand dominated channel deposits, and the other two are mudstone-sandstone, and mudstone dominated overbank deposits. The initial sedimentation in the region was in channels and frequently/extensively flooded overbank areas of a meandering/anastomosing river system. Activities along basement structures sometimes caused upheaval of the basin so that the streams got incised, and overbank areas rose up beyond the reach of flood waters. As a result, the fluvial sedimentation in these upland areas ceased, the sediments that had already deposited there were subjected to extensive pedogenesis, and occasionally reworked and redistributed by sheet flows and shallow channels. The channel pattern in the region gradually changed to braided type due to channel adjustments in response to rejuvenated tectonic activities and monsoon intensification in the hinterland. These factors caused increased influx of coarser sediments in the channels, which led to gradual steepening of the channel, and once the steepening crossed the threshold, the channel changed from meandering to braided type. Interpretation of our results is contrary to the general belief that Siwalik fluvial system changed from meandering streams to braided streams during the Middle Siwalik times, and the fluvial system in the studied part of the Siwalik basin underwent this change much earlier, during the sedimentation of Lower Siwalik.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-09-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44024482","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Age, petrogenesis, and tectonic implications of the late Permian magmatic rocks in the Middle Gobi volcanoplutonic Belt, Mongolia","authors":"Ariuntsetseg Ganbat,&nbsp;Tatsuki Tsujimori,&nbsp;Laicheng Miao,&nbsp;Inna Safonova,&nbsp;Daniel Pastor-Galán,&nbsp;Chimedtseren Anaad,&nbsp;Shogo Aoki,&nbsp;Kazumasa Aoki,&nbsp;Munkhnasan Chimedsuren","doi":"10.1111/iar.12457","DOIUrl":"10.1111/iar.12457","url":null,"abstract":"<p>The Mongol–Okhotsk Belt, the youngest segment of Central Asian Orogenic Belt, was formed by the evolution and closure of the Mongol–Okhotsk Ocean. The oceanic closure formed two volcanoplutonic belts: Selenge Belt in the north and the Middle Gobi Belt in the south (in present day coordinates). However, the origin and tectonic evolution of the Mongol–Okhotsk Belt in general, and the origin and formation age of the Middle Gobi Belt in particular, remain enigmatic. To better understand the history of the magmatic activity in the Middle Gobi Belt, we conducted geochemical, U–Pb geochronological, zircon Hf, and whole-rock Nd isotopic analyses of samples from the Mandalgovi volcanoplutonic suite, the major component of the Middle Gobi Belt. Our results show that the plutonic rock consists of ~285 Ma gabbro, ~265 Ma biotite-granite and ~250 Ma hornblende-granodiorite. The volcanic counterpart is represented by a Permian Sahalyn gol rhyolite and ~247 Ma Ikh khad andesite. The geochemical compositions of biotite-granite and hornblende-granodiorite indicate that their precursors were metagraywacke and amphibolite, respectively. They are characterized by positive whole-rock ε_Nd(<i>t</i>) and zircon ε_Hf(<i>t</i>) values, indicating juvenile protoliths. The gabbro was derived by partial melting of a metasomatized lithospheric mantle source in a supra-subduction setting. The biotite-granite and Sahalyn gol rhyolite are formed by remelting of sediments in an inter-arc extension setting. Later the hornblende-granite and Ikh khad volcanic were emplaced at a volcanic arc formed by the subduction of the Mongol–Okhotsk Ocean. We conclude that the magmatic rocks of the Middle Gobi Belt formed in an active continental margin setting. Considering the consistent distribution of coeval arc-derived magmatic formations along the southern margin of the Mongol–Okhotsk Belt, the oceanic basin was closed in a relatively simultaneous manner.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-08-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45224460","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Stratigraphic reconstruction of the lower–middle Miocene Goto Group, Nagasaki Prefecture, Japan","authors":"Shoichi Kiyokawa,&nbsp;Masaru Yasunaga,&nbsp;Takanori Hasegawa,&nbsp;Ayako Yamamoto,&nbsp;Daisaku Kaneko,&nbsp;Yuta Ikebata,&nbsp;Noriko Hasebe,&nbsp;Yukiyasu Tsutsumi,&nbsp;Mami Takehara,&nbsp;Kenji Horie","doi":"10.1111/iar.12456","DOIUrl":"10.1111/iar.12456","url":null,"abstract":"<p>The Goto Islands are located at the westernmost tip of the Japan archipelago, and preserve a lower–middle Miocene sedimentary sequence deposited during rifting of the continental margin and opening of the Sea of Japan. The stratigraphy of the Goto Group and new K–Ar, fission-track, and U–Pb age data were used to determine the initial conditions of rifting in southwest Japan. The thickness of the Goto Group is 2000–3000 m. The lower unit (ca. 22–17.6 Ma) consists of thick, greenish, volcaniclastic rocks with basaltic volcanic material, representing the initial stages of continental rifting. The middle unit (ca. −17.6 Ma) consists of alternating sandstones and shales deposited in lacustrine and meandering fluvial environments in a syn-rift sedimentary basin during a period of volcanic activity. The upper unit (ca. 17.6–16.8 Ma) consists of thick sandstones of fluvial–deltaic facies that were deposited during rapid subsidence at the continental margin. This unit was deposited by a large fluvial system that flowed into the Sea of Japan. These sequences contain relatively cooler to warmer flora (Daijima-type) and record the warm period of the Miocene Climatic Optimum. The Goto felsic volcanic rocks (16.8 ~ 15.4 Ma) unconformably overlie the Goto Group, and granitic magmatism (ca. 16–14.5 Ma) occurred after sedimentation of the Goto Group. The widespread lacustrine, meandering–braided fluvial, and vast deltaic systems of the Goto Group, and felsic volcanism, were formed due to rapid subsidence that produced a horst-and-graben basin during the early stages of rifting of a volcanic arc along the eastern margin of Eurasia. These events occurred from 22.0 to 16.8 Ma before and during the formation of the Sea of Japan.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-08-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12456","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45043680","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Multistage zircon growth recording polyphase metamorphic evolution caused by pulsed granitoid intrusions into a low-P/T type metamorphic belt: P–T–D–t evolution of migmatites in the Ryoke belt, southwest Japan","authors":"Tetsuo Kawakami,&nbsp;Tomoe Ichino,&nbsp;Keiichi Kazuratachi,&nbsp;Shuhei Sakata,&nbsp;Kota Takatsuka","doi":"10.1111/iar.12454","DOIUrl":"10.1111/iar.12454","url":null,"abstract":"<p>We report contrasting pressure–temperature–time (<i>P–T–t</i>) paths of migmatites developed in the highest-grade metamorphic zone (Grt–Crd zone) and the contact metamorphic zone (Crd–Kfs zone) of the Mikawa area, Ryoke belt, southwest Japan to discuss the complex <i>P–T–D–t</i> evolution of the middle crust that experienced pulsed granitoid intrusions. In the Grt–Crd zone, sillimanite-grade high-<i>T</i> metamorphic condition prevailed from ca. 97 to 87 Ma, followed by cooling to ~500 °C, ~4 kbar. The intrusion of gneissose granitoids below the Grt–Crd zone isobarically reheated the Grt–Crd zone rocks again to the sillimanite-grade high-<i>T</i> condition at ca. 84 Ma. This was followed by ca. 71–70 Ma contact metamorphism. Ductile deformation that formed and folded the foliation of migmatites started before ca. 89 Ma and continued at least until ca. 84 Ma in the Grt–Crd zone. On the other hand, ca. 74 Ma age of the Crd–Kfs zone migmatite developed around the Inagawa Granodiorite in addition to ca. 70 Ma age of a syn-tectonic pegmatite vein revealed that the intrusion of “75–69 Ma granitoids” caused partial melting and locally triggered low-strain ductile deformation in their contact aureoles. Comparison with other areas of the Ryoke belt suggests that plutono-metamorphic evolution of the Mikawa and Aoyama areas are similar with each other in that ca. 80 Ma reheating events (i.e., contact metamorphism) are observed, while absence of separate reheating event postdating peak metamorphism in the Yanai area is a rather uncommon feature in the Ryoke belt.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-07-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43734689","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Trace-element composition of zircon in Kofu and Tanzawa granitoids, Japan: Quantitative indicator of sediment incorporated in parent magma","authors":"Yusuke Sawaki,&nbsp;Hisashi Asanuma,&nbsp;Shuhei Sakata,&nbsp;Mariko Abe,&nbsp;Takeshi Ohno","doi":"10.1111/iar.12455","DOIUrl":"10.1111/iar.12455","url":null,"abstract":"<p>Zircon is one of the most important minerals in geochronologic research. Isotopic ratios and trace elements in zircons are expected to reflect those of their parent magmas. Many geochemical researchers have proposed various discrimination diagrams for zircon to indicate tectonic setting and to identify source rock. Because most detrital zircons accumulated at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. In our research, we focused on sediment involvement during granitoid formation and tried to identify trace-element compositions in zircon that are sensitive to variation in sediment incorporation. To accomplish this, we examined trace-element compositions of both the granitoids and the included zircons in the Kofu granitic complex and the Tanzawa tonalitic plutons in Japan. Among the high-field-strength elements (Th, U, Ta, Nb, Hf, and rare earth elements), only Nb and Ta concentrations in the granitoids increased as the rate of sediment contribution increased. However, the zircon did not show such trends in Nb and Ta content. Zircon Y and P contents exhibited a positive correlation, indicating that xenotime substitution occurs to some extent. Because P exists as pentavalent ions in igneous systems, its presence likely affects the concentrations of pentads in zircon. When we divided the Nb and Ta contents by the P content, it became clear that zircon Nb/P and Ta/P ratios increase depending on sediment involvement. While some exceptions exist, we found that zircon Yb/Gd ratios also respond to sediment involvement. Our data further demonstrated that zircons in granitoids with significant sediment incorporation are characterized by low Ce/P contents, which is partly attributable to monazite crystallization before zircon saturation. This study demonstrates that combining these element ratios is useful for indicating sediment incorporation.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43120692","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Middle Holocene relative sea-level changes and vertical tectonic crustal movements on Shikoku Island near the Nankai Trough, Japan","authors":"Tatsuhiko Yamaguchi,&nbsp;Futoshi Nanayama,&nbsp;Toshimichi Nakanishi,&nbsp;Tomohiro Tsuji,&nbsp;Michiharu Ikeda,&nbsp;Yasuo Kondo,&nbsp;Michiko Miwa,&nbsp;Yohei Hamada","doi":"10.1111/iar.12452","DOIUrl":"10.1111/iar.12452","url":null,"abstract":"<p>The Philippine Sea plate subducts beneath the Eurasia plate at the Nankai Trough, northwestern Pacific, causing crustal deformation, mega-thrust earthquakes, and tsunami events. Shikoku Island, 150 km northwest of the trough, experiences both coseismic and interseismic deformation. Coastal sediments potentially record vertical crustal movements as relative sea level (RSL) changes. We studied sedimentary facies and microfossil ostracodes in core SKM from southwestern Shikoku Island for evidence of middle Holocene tsunami events and deformation. The core sediments included nine event layers corresponding to storm or tsunami events. Using modern analog techniques, we estimated RSLs from the ostracode assemblages of core SKM and 13 other cores from Shikoku Island and the surrounding region. Then, we subtracted RSL changes due to glacio-hydro isostatic adjustment from the estimated RSLs to estimate vertical tectonic movement rates in these cores between 8.6 and 4.7 ka. The inferred RSL changes suggest that the Sukumo site has experienced both uplift and subsidence since 8.6 ka. Before 6.6 ka, rates of tectonic crustal movement were higher than the modern-day rate, and its spatial distribution also differed. After 6.6 ka, tectonic crustal movement showed a similar spatial pattern and occurred at rates close to the modern day interseismic rate. The spatial pattern and rates of tectonic crustal movement could be caused by changes in rupture areas between the Eurasia and the Philippine Sea plates beneath the Shikoku Island and in stress condition of the asthenosphere. Some of the vertical displacements can be explained by the movements of local active faults.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-06-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49277730","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"An obelisk-shaped granitoid tower at Mt. Jizogadake in the Southern Alps of Japan: A 3-D morphological study","authors":"Masahiro Chigira","doi":"10.1111/iar.12449","DOIUrl":"10.1111/iar.12449","url":null,"abstract":"<p>Well-developed tors and associated boulder fields are present in exposed granitoid pluton, but their geomorphological origins have not been fully addressed. Although the tor-like features are commonly attributed to the orthogonal joint system along which weathering proceeds and the weathering materials are removed, the majority of previous studies lack three-dimensional (3-D) morphological observations. One of the well-known granitoid rock towers in Japan, namely, the obelisk at Mt. Jizogadake in the Japanese Southern Alps, was investigated using unmanned air vehicles (UAVs). A 3-D observation of the granitoid obelisk found that it is shaped by columnar joints with an undulation along a long axis and irregularly shaped cross-sections. Because of these features and intersection angles between the rock columns and exposure surfaces, the obelisk exhibits a peculiar morphology appearing as a heap of rock blocks when observed from the ground. The columnar jointing might have occurred in a marginal zone of the granitoid pluton due to faster cooling rates. The 3-D observations also confirmed the columnar joints formation was predated by high-angle, planar joints. The columnar joints divide the rock mass between those planar joints. This may imply that the high-angle joints would have formed probably during cooling under the presence of tectonic and thermal stresses.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-06-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62641144","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}