| 1. |
- Abdollahi, Somayeh, et al.
(författare)
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3D joint inversion of gravity data and Rayleigh wave group velocities to resolve shear-wave velocity and density structure in the Makran subduction zone, south-east Iran
- 2019
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Ingår i: Journal of Asian Earth Sciences. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1367-9120 .- 1878-5786. ; 173, s. 275-290
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we developed a method to invert jointly Rayleigh wave group velocities and gravity anomalies for velocity and density structure of the lithosphere. We applied the method to the Makran accretionary prism, SE Iran. The reason for using different data sets is that each of these data sets is sensitive to different parameters. Surface wave group velocities are sensitive mainly to shear wave velocity distribution in depth but do not well resolve density variations. Therefore, joint inversion with gravity data increases the resolution of density distribution. Our approach differs from others mainly in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for the properties of only four layers: thickness, P- and S-wave velocities and densities and their vertical gradients in sediments, upper-crust, lower-crust and upper mantle. The method is applied first to synthetic models in order to demonstrate its usefulness. We then applied the method to real data to investigate the lithosphere structure beneath the Makran. The resulting model shows that Moho depth increases from Oman Sea (18-33 km) and Makran fore-arc (33-37 km) to the volcanic-arc (44-46 km). The crustal density is high in the Oman Sea as should be expected for the oceanic crust. We also find a high-velocity anomaly in the upper mantle under the Oman Sea corresponding to the subducting slab. The crust under the fore-arc, volcanic-arc and back-arc settings of Makran subduction zone is characterized by low-velocity zones.
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| 2. |
- Anand, Rajagopal, et al.
(författare)
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Neoarchean crustal growth by accretionary processes: Evidence from combined zircon–titanite U–Pb isotope studies on granitoid rocks around the Hutti greenstone belt, eastern Dharwar Craton, India
- 2014
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier. - 1367-9120 .- 1878-5786. ; 79, s. 72-85
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Tidskriftsartikel (refereegranskat)abstract
- The Neoarchean Hutti greenstone belt hosts mesothermal gold deposits and is surrounded by granitoid rocks on all sides. Combined U–Pb dating of zircon and titanite from the granitoid rocks constrains their emplacement history and subsequent geologic evolution. The Golapalli and Yelagatti granodiorites occurring to the north of the Hutti greenstone belt were emplaced at 2569 ± 17 Ma. The Yelagatti granodiorite yielded a younger titanite age of 2530 ± 6 Ma which indicates that it was affected by a post-crystallization thermal event that exceeded the titanite closure temperature. The western granodiorites from Kardikal have identical titanite and zircon ages of 2557 ± 6 Ma and 2559 ± 19 Ma, respectively. The eastern Kavital granodiorites yielded titanite ages of 2547 ± 6 Ma and 2544 ± 24 Ma which are identical to the published U–Pb zircon SHRIMP ages. These ages imply that the granitoid rocks surrounding the Hutti greenstone belt were formed as discrete batholiths within a short span of ca. 40 Ma between 2570 Ma and 2530 Ma ago. They were juxtaposed by horizontal tectonic forces against the supracrustal rocks that had formed in oceanic settings at the end of the Archean. The first phase of gold mineralization coincided with the last phase of granodiorite intrusion in the Hutti area. A metamorphic overprint occurred at ca. 2300 Ma ago that reset the Rb–Sr isotope system in biotites and possibly caused hydrothermal activity and enrichment of Au in the ore lodes. The eastern Dharwar Craton consists of quartz monzodiorite–granodiorite–granite suites of rocks that are younger than the greenstone belts that are older than ~2650 Ma reported from earlier studies. The granitoid magmatism took place between 2650 and 2510 Ma ago indicating accretionary growth of the eastern Dharwar Craton.
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| 3. |
- Aydin, Faruk, et al.
(författare)
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Temporal, geochemical and geodynamic evolution of the Late Cretaceous subduction zone volcanism in the eastern Sakarya Zone, NE Turkey : Implications for mantle-crust interaction in an arc setting
- 2020
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier. - 1367-9120 .- 1878-5786. ; 192
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Tidskriftsartikel (refereegranskat)abstract
- The Late Cretaceous Artvin volcanic rocks (LCAVs) from the eastern Sakarya zone (ESZ) of NE Turkey are composed of mafic/basaltic (S1-Çatak and S2-Çağlayan) and felsic/acidic (S1-Kızılkaya and S2-Tirebolu) rock types that occurred in two successive stages: (i) first stage (S1: Turonian–Early Santonian) and (ii) second stage (S2: Late Santonian–Campanian). Clinopyroxene thermobarometric results point that the S2-Çağlayan basaltic rocks have crystallised at higher temperatures and under deeper crustal conditions than those of the S1-Çatak basaltic rocks.The LCAVs show a wide compositional spectrum, ranging from tholeiite to calc-alkaline/shoshonite and are typically represented by a geochemical composition resembling subduction-related arc rocks although the 87Sr/86Sr(i) (0.7044–0.7071) and ɛNd(i) values (−0.63 to + 3.47) as well as 206Pb/204Pb(i) (18.07–18.56), 207Pb/204Pb(i) (15.57–15.62) and 208Pb/204Pb(i) (37.12–38.55) ratios show very limited variation. The parent magmas of the S1-Çatak and S2-Çağlayan mafic volcanic rocks were derived from underplated basaltic melts that originated by partial melting of metasomatised spinel lherzolite and spinel-garnet lherzolite, respectively. It is proposed that the compositions of the S1-Kızılkaya (mainly dacitic) and S2-Tirebolu (rhyolitic to trachytic) felsic rocks were particularly controlled by metasomatised mantle–crust interaction and MASH zone + shallow crustal fractionation processes. Our data, together with data from previous studies, suggest that the S1- and S2-mafic and felsic rocks of the LCAVs (~95–75 Ma) are the products of two-stage volcanic event that took place during the northward subduction of the northern Neotethys Ocean (NNO).
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| 4. |
- Bagherbandi, Mohammad
(författare)
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A comparison of three gravity inversion methods for crustal thickness modelling in Tibet plateau
- 2012
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 43:1, s. 89-97
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Tidskriftsartikel (refereegranskat)abstract
- Crustal thickness can be determined by gravimetric methods based on different assumptions, e.g. by isostatic hypotheses. Here we compare three gravimetric inversion methods to estimate the Moho depth. Two Moho models based on the Vening Meinesz-Moritz hypothesis and one by using Parker-Oldenburg's algorithm, which are investigated in Tibet plateau. The results are compared with CRUST2.0, and it will be presented that the estimated Moho depths from the Vening Meinesz-Moritz model will be better than the Parker-Oldenburg's algorithm.
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| 5. |
- Bagherbandi, Mohammad
(författare)
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Combination of seismic and an isostatic crustal thickness models using Butterworth filter in a spectral approach
- 2012
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 59:SI, s. 240-248
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Tidskriftsartikel (refereegranskat)abstract
- In this study, using Butterworth filter a combined crustal thickness model based on seismic and isostatic-gravimetric models is presented in a spectral domain. Vening Meinesz-Moritz isostatic model and a seismic model which obtained from sparse seismic data are two models used in this study. The filter used helps to join two models without any jump in the overlap degree in the spectral domain. The main motivations of this study are (a) presenting a higher resolution for the crustal thickness and (b) removing non-isostatic effects from the isostatic model. The result obtained from the combined model is a synthetic Earth crustal model up to degree 180 (equivalent resolution 1° × 1°). In spite of the differences in the some parts of the Earth between the seismic and isostatic-gravimetric models, the test computations show a satisfactory agreement between the results provided. Numerical results show that this method of combination agrees with the seismic crustal thickness (about 2.0. km rms difference).
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| 6. |
- Bercovici, Antoine, et al.
(författare)
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Terrestrial paleoenvironment characterization across the Permian-Triassic boundary in South China
- 2015
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 98, s. 225-246
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Tidskriftsartikel (refereegranskat)abstract
- Well-preserved marine fossils in carbonate rocks permit detailed studies of the end-Permian extinction event in the marine realm. However, the rarity of fossils in terrestrial depositional environments makes it more challenging to attain a satisfactory degree of resolution to describe the biotic turnover on land. Here we present new sedimentological, paleontological and geochemical (X-ray fluorescence) analysis from the study of four terrestrial sections (Chahe, Zhejue, Mide and Jiucaichong) in Western Guizhou and Eastern Yunnan (Yangtze Platform, South China) to evaluate paleoenvironmental changes through the Permian-Triassic transition. Our results show major differences in the depositional environments between the Permian Xuanwei and the Triassic Kayitou formations with a change from fluvial-lacustrine to coastal marine settings. This change is associated with a drastic modification of the preservation mode of the fossil plants, from large compressions to small comminuted debris. Plant fossils spanning the Permian-Triassic boundary show the existence of two distinct assemblages: In the Xuanwei Formation, a Late Permian (Changhsingian) assemblage with characteristic Cathaysian wetland plants (mainly Gigantopteris dictyophylloides, Gigantonoclea guizhouensis, G. nicotianaefolia, G. plumosa, G. hallei, Lobatannularia heinanensis, L. cathaysiana, L. multifolia, Annularia pingloensis, A. shirakii, Paracalamites stenocostatus, Cordaites sp.) is identified. In the lowermost Kayitou Formation, an Early Triassic (Induan) Annalepis-Peltaspermum assemblage is shown, associated with very rare, relictual gigantopterids. Palynological samples are poor, and low yield samples show assemblages almost exclusively represented by spores. A similar to 1 m thick zone enriched in putative fungal spores was identified near the top of the Xuanwei Formation, including diverse multicellular forms, such as Reduviasporonites sp. This interval likely corresponds to the PTB "fungal spike" conventionally associated with land denudation and ecosystem collapse. While the floral turnover is evident, further studies based on plant diversity would be required in order to assess contribution linked to the end-Permian mass extinction versus local paleoenvironmental changes associated with the transition between the Xuanwei and Kayitou formations. (C) 2014 Elsevier Ltd. All rights reserved.
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| 7. |
- Eshagh, Mehdi, 1977-, et al.
(författare)
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Isostatic GOCE Moho model for Iran
- 2017
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 138, s. 12-24
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Tidskriftsartikel (refereegranskat)abstract
- One of the major issues associated with a regional Moho recovery from the gravity or gravity-gradient data is the optimal choice of the mean compensation depth (i.e., the mean Moho depth) for a certain area of study, typically for orogens characterised by large Moho depth variations. In case of selecting a small value of the mean compensation depth, the pattern of deep Moho structure might not be reproduced realistically. Moreover, the definition of the mean compensation depth in existing isostatic models affects only low-degrees of the Moho spectrum. To overcome this problem, in this study we reformulate the Sjöberg and Jeffrey’s methods of solving the Vening-Meinesz isostatic problem so that the mean compensation depth contributes to the whole Moho spectrum. Both solutions are then defined for the vertical gravity gradient, allowing estimating the Moho depth from the GOCE satellite gravity-gradiometry data. Moreover, gravimetric solutions provide realistic results only when a priori information on the crust and upper mantle structure is known (usually from seismic surveys) with a relatively good accuracy. To investigate this aspect, we formulate our gravimetric solutions for a variable Moho density contrast to account for variable density of the uppermost mantle below the Moho interface, while taking into consideration also density variations within the sediments and consolidated crust down to the Moho interface. The developed theoretical models are applied to estimate the Moho depth from GOCE data at the regional study area of the Iranian tectonic block, including also parts of surrounding tectonic features. Our results indicate that the regional Moho depth differences between Sjöberg and Jeffrey’s solutions, reaching up to about 3 km, are caused by a smoothing effect of Sjöberg’s method. The validation of our results further shows a relatively good agreement with regional seismic studies over most of the continental crust, but large discrepancies are detected under the Oman Sea and the Makran subduction zone. We explain these discrepancies by a low quality of seismic data offshore.
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| 8. |
- Eshagh, Mehdi, et al.
(författare)
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Towards sub-lithospheric stress determination from seismic Moho, topographic heights and GOCE data
- 2016
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 129, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- Sub-lithospheric stresses can be estimated by analysis of gravity field measurements. Depending on the measured gravimetric quantity, different methods can be employed to estimate those sub-lithospheric stresses. Here, we further develop the Runcorn's theory for estimation of mantle stresses (1967) such that a Moho model and full topographic information are used to recover the function from which the stress can be computed by taking derivatives northwards and eastwards. We develop new integral equations for such a purpose and recover this function by solving those integral equations locally over the Indo-Pak (India-Pakistan) region from (1) a gravimetric Moho model computed from the SRTM (Shuttle Radar Topography Mission) and the Earth gravity model EGM2008, (2) SRTM and the seismic Moho model of CRUST1.0 and (3) data and measurements of the GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission. Finally, we perform a joint inversion of seismic and GOCE data for the same purpose. The numerical results show that the use of a seismic Moho model recovers information about the stress field which is not seen in the results derived from a gravimetric Moho model. A combination of the seismic Moho model, SRTM and GOCE yields a better stress field than that of either the seismic and/or gravimetric data alone. The magnitudes of the sub-lithospheric stress are computed from the shear stress components over the area and good agreement is seen between the recovered combined stress field, the regional tectonic boundaries and the seismicity of the World Stress Map 2008 database.
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| 9. |
- Ghasemi, A, et al.
(författare)
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A new tectonic scenario fro the Sanandaj-Sirjan Zone (Iran)
- 2006
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 26:6, s. 683-693
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Tidskriftsartikel (refereegranskat)abstract
- Recent geochemical studies of volcanic rocks forming part of the ophiolites within the Zagros and Naien-Baft orogen indicate that most of them were developed as supra-subduction ophiolites in intra-oceanic island arc environments. Intra-oceanic island arcs and ophiolites now forming the Naien-Baft zone were emplaced southwestward onto the northeastern margin of the South Sanandaj-Sirjan Zone, while those now in the High Zagros were emplaced southwestward onto the northern margin of Arabia. Thereafter, subduction continued on opposite sides of the remnant oceans. The floor of Neo-Tethys Ocean was subducted at a low angle beneath the entire Sanandaj-Sirjan Zone, and the floor of the Naien-Baft Ocean was subducted beneath the Central Iranian Micro-continent. The Naien-Baft Ocean extended into North-West Iran only temporarily. This failed ocean arm (between the Urumieh-Dokhtar Magmatic Assemblage and the main Zagros Thrust) was filled by thick Upper Triassic-Upper Jurassic sediments. The Naien-Baft Ocean finally closed in the Paleocene and Neo-Tethys closed in the Early to Middle Eocene. After Arabia was sutured to Iran. the Urumieh-Dokhtar Magmatic Assemblage recorded slab break-off in the Middle Eocene.
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| 10. |
- Ledneva, Galina V., et al.
(författare)
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Permo"Triassic hypabyssal mafic intrusions and associated tholeiitic basalts of the Kolyuchinskaya Bay, Chukotka (NE Russia) : Links to the Siberian LIP
- 2011
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 40:3, s. 737-745
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Tidskriftsartikel (refereegranskat)abstract
- In order to test tectonic hypotheses regarding the evolution of the Arctic Alaska-Chukotka microplate prior to the opening of the Amerasian basin, we investigated rocks exposed near Kolyuchinskaya Bay, eastern Chukotka. Hypabyssal mafic rocks and associated basaltic flows enclose terrigenous sediments, minor cherts and limestones in pillow interstices. The hypabyssal mafic rock yields a U-Pb zircon age of 252 +/- 4 Ma and indicates intrusion of basic magma at the Permo-Triassic boundary, contemporaneous with voluminous magmatism of the Siberian large igneous province (LIP). The lava flows and hypabyssal mafic rocks of the Kolyuchinskaya Bay region have trace elements, Sm-Nd and Rb-Sr isotope compositions identical to the tholeiitic flood basalts of the main plateau stage of the Siberian LIP, but differ from the latter in the major-element variations. We conclude that compositional variations in the hypabyssal rocks studied reflect their generation in an extensional environment that might be related to the Siberian super-plume activity at the time. Although the genetic and temporal links between intrusive mafic rocks and lavas are not well proved, compositional variations of the eruptive rocks still indicate their generation in an extensional environment.
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| 11. |
- Liang, Yue, et al.
(författare)
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Late Ordovician lingulid brachiopods from the Pingliang Formation (Shaanxi Province, North China) : Morphological and ecological implications
- 2024
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier. - 1367-9120 .- 1878-5786. ; 263
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Tidskriftsartikel (refereegranskat)abstract
- Brachiopods first appeared in the early Cambrian and persist till present. They are one of the main lineages of marine invertebrates that diversified throughout the Paleozoic and reached their maximum diversity of high-rank taxonomy during the Ordovician. During this time interval, brachiopods were mainly dominated by the articulated Orthida and Strophomenida, which represent major components of the Paleozoic Evolutionary Fauna; however, the inarticulated lingulid brachiopods (Order Lingulida) were comparatively less abundant and received less attention during this time period. Here, we report a new record of the lingulid brachiopod Anomaloglossa porca from the Upper Ordovician (Sandbian) Pingliang Formation of the Xilinggou Section, Shaanxi Province, North China. Collected specimens are preserved as calcium phosphatic shells with highly mineralization which preserve detailed morphology and shell ornamentation of both ventral and dorsal valves. The new occurrence of A. porca extends its paleogeographic distribution from Gondwana and Tarim to North China Platform. Moreover, comprehensive geometric morphometric analysis of A. porca is performed and the results indicate that both the shell shape and pseudointerarea are very close to the recent infaunal lingulids. If interpreted correctly, the fossils represent the first example of infaunal lifestyles achieved by Ordovician lingulids from North China, exhibiting the ecological complexities of the Late Ordovician benthos composed of epibenthos and infaunas as well.
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| 12. |
- Löwemark, Ludvig, et al.
(författare)
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Normalizing XRF-scanner data : A cautionary note on the interpretation of high-resolution records from organic-rich lakes
- 2011
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 40:6, s. 1250-1256
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Tidskriftsartikel (refereegranskat)abstract
- X-ray fluorescence (XRF) scanning of unlithified, untreated sediment cores is becoming an increasingly common method used to obtain paleoproxy data from lake records. XRF-scanning is fast and delivers high-resolution records of relative variations in the elemental composition of the sediment. However, lake sediments display extreme variations in their organic matter content, which can vary from just a few percent to well over 50%. As XRF scanners are largely insensitive to organic material in the sediment, increasing levels of organic material effectively dilute those components that can be measured, such as the lithogenic material (the closed-sum effect). Consequently, in sediments with large variations in organic material, the measured variations in an element will to a large extent mirror the changes in organic material. It is therefore necessary to normalize the elements in the lithogenic component of the sediment against a conservative element to allow changes in the input of the elements to be addressed. In this study we show that Al, which is the lightest element that can be measured using the Wax XRF-scanner, can be used to effectively normalize the elements of the lithogenic fraction of the sediment against variations in organic content. We also show that care must be taken when choosing resolution and exposure time to ensure optimal output from the measurements.
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| 13. |
- Löwemark, Ludvig, et al.
(författare)
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The Tienchi Pond on Lanyu Island (Western Pacific) : Lake formation and potential as environmental archive
- 2015
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 114, s. 435-446
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Tidskriftsartikel (refereegranskat)abstract
- The formation, evolution, and potential as a climate archive of Tienchi Pond was evaluated using geomor-phology, sedimentology, geochemistry, and AMS radiocarbon measurements of tree ring cores. Tienchi Pond is a small ephemeral lake situated on the subtropical Lanyu Island in the Western Pacific Ocean about 90 km east of southern Taiwan. The lake is situated on a mountain ridge and is today characterized by numerous dead trees and stumps. The position at the boundary between the two main components of the SE Asian monsoon system, the Asian landmass and the Western Pacific, makes this lake particularly interesting as a potential natural climate archive. Since previous studies have shown that the bedrock underlying the lake formed over 2 my ago, a volcanic crater origin was ruled out. Rather, element ratios of K/Ti and Ca/Fe together with Pb-210 dating suggest a recent origin for the lake, probably as the result of a natural damming event in the southern, narrower, part of the lake basin, or as a result of increased precipitation at the end of the Little Ice Age (or a combination of both). Radiocarbon measurements on a dead tree near the lake perimeter show C-14 levels of more than 1.7% modern radiocarbon near the bark, suggesting that the tree died in the early 1970s. In contrast, by correlating a large number of radiocarbon measurements to a northern hemisphere reference curve, it was shown that trees closer to the center of the lake died in the early 20th century. This agrees with a scenario where a small lake formed by initial damming, and gradually grew larger through erosion of the surrounding shore lines caused by rapid lake level oscillations.
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| 14. |
- Meng, Qingpeng, et al.
(författare)
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Time constraints on the inversion of the tectonic regime in the northern margin of the North China Craton : Evidence from the Daqingshan granites
- 2014
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 79, s. 246-259
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Tidskriftsartikel (refereegranskat)abstract
- The Daqingshan granites are located in a late Mesozoic tectono-magmatic belt at the northern margin of the North China Craton (NCC), and include the Deshengying, Xinisubei, Gulouban, and Kuisu plutons. Ion probe U-Pb zircon dating indicates that the granites were emplaced at 131 +/- 1, 140 +/- 4, 145 +/- 1, and 142 +/- 2 Ma, respectively. All of the granites are alkali- and potassium-rich, with high SiO2 (73.2-76.7 wt.%), K2O (4.50-5.57 wt.%), Na2O (3.60-4.93 wt.%), and K2O/Na2O (0.99-1.49), and low Al2O3 (12.3-14.5 wt.%), CaO (0.45-0.79 wt.%), and MgO (0.12 wt.%). The granites are light rare earth element enriched a La/YbIN = 5.6-48.7). The Xinisubei and Gulouban monzogranites and the Kuisu mylonitic monzogranite have small Eu anomalies (8Eu = 0.65-1.23), low Zr + Nb + Ce + Y (132-321 ppm), and exhibit a negative correlation between P2O5 and SiO2 contents, which are characteristic of highly fractionated I-type granites with a post-collisional origin. The Deshengying monzogranite is distinctive in being an aluminous A-type granite as evidenced by high 10,000 x Ga/AI (>2.6) and Zr + Nb + Ce + Y (312-532 ppm), low Ba and Sr, marked negative Eu anomalies (8Eu = 0.08-0.20), strong Ba, Sr, P, and Ti depletions, and an absence of alkali minerals. This granite was probably produced by partial melting of continental crust heated by hot mantle-derived magmas during crustal extension. The Deshengying monzogranite represents a post-kinematic pluton emplaced into the Daqingshan fold-and-thrust belt, whereas the Kuisu mylonitic monzogranite is a syn-kinematic pluton intruded along the Hohhot detachment fault. It is evident that the Daqingshan area experienced a change from a compressional to an extensional tectonic regime during 145-140 Ma. The post-orogenic collapse may have resulted in extension of the upper continental crust. Subsequently, as the thrust-detachment system became inactive, the lower crust of the NCC underwent modification and melting from 131 Ma. We conclude that the Early Cretaceous tectonic evolution of the Daqingshan area was caused by post-orogenic collapse and melting of the lower crust of the NCC. Delamination of the lower crust in the northern NCC resulted in crustal extension and asthenospheric upwelling, which produced A-type granites. As such, melting of the lower crust in the northwestern part of the NCC took place as early as the late Mesozoic.
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| 15. |
- Mostafanejad, Akram, et al.
(författare)
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3-D velocity structure of Damavand volcano, Iran, from local earthquake tomography
- 2011
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 42:6, s. 1091-1096
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Tidskriftsartikel (refereegranskat)abstract
- Damavand volcano is a large intraplate Quaternary composite cone overlying the active fold and thrust belt of the Central Alborz Mountains in northern Iran. In this study, we present the first 3-D P-wave velocity model of the upper crust for the Central Alborz region using local earthquake data provided by the Iranian Seismic Telemetry Network. The final P-wave velocity model reveals several high and low velocity anomalies in the upper 30 km of the crust. A low velocity zone parallel to the main faulting system is imaged at a depth of 15 km. A relatively high velocity body to the north of the Damavand summit down to a depth of 20 km is resolved, which may represent the elderly crystallized magma chamber. Right below the Damavand cone, a low velocity area to the depth of 7 km can be interpreted as a shallow magma chamber.
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| 16. |
- Ng, S.W.P., et al.
(författare)
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Late Triassic granites from Bangka, Indonesia: a continuation of the Main Range granite province of the South-East Asian Tin Belt.
- 2017
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 138, s. 548-561
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Tidskriftsartikel (refereegranskat)abstract
- The South-East Asian Tin Belt is one of the most tin-productive regions in the world. It comprises three north-south oriented granite provinces, of which the arc-related Eastern granite province and the collision-related Main Range granite province run across Thailand, Singapore, and Indonesia. These tin-producing granite provinces with different mineral assemblages are separated by Paleo-Tethyan sutures exposed in Thailand and Malaysia. The Eastern Province is usually characterised by granites with biotite ± hornblende. Main Range granites are sometimes characterised by the presence of biotite ± muscovite. However, the physical boundary between the two types of granite is not well-defined on the Indonesian Tin Islands, because the Paleo-Tethyan suture is not exposed on land there. Both hornblende-bearing (previously interpreted as I-type) and hornblende-barren (previously interpreted as S-type) granites are apparently randomly distributed on the Indonesian Tin Islands. Granites exposed on Bangka, the largest and southernmost Tin Island, no matter whether they are hornblende-bearing or hornblende-barren, are geochemically similar to Malaysian Main Range granites. The average ɛNd(t) value obtained from the granites from Bangka (average ɛNd(t) = −8.2) falls within the range of the Main Range Province (−9.6 to −5.4). These granites have SIMS zircon U-Pb ages of ca. 225 Ma and ca. 220 Ma, respectively that are both within the period of Main Range magmatism (∼226–201 Ma) in the Peninsular Malaysia. We suggest that the granites exposed on Bangka represent the continuation of the Main Range Province, and that the Paleo-Tethyan suture lies to the east of the island.
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| 17. |
- Nuannin, Paiboon, et al.
(författare)
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Spatial and temporal characteristics of aftershocks of the December 26, 2004 and March 28, 2005 earthquakes off NW Sumatra
- 2012
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 46, s. 150-160
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Tidskriftsartikel (refereegranskat)abstract
- Two aftershock series following the December 26, 2004 (M-w = 9.0) and the March 28, 2005 (M-w = 8.6) earthquakes off coast of NW Sumatra are studied. Deduced space heterogeneity of b-values, in the Gutenberg-Richter formula, and p-values, in the Omori law, show significant variations. General agreement is found between areas of high b and areas of significant slip predicted by different source models presented by other workers. Correlation with high p-values does exist, however, is less pronounced. The largest aftershocks occur during or near time periods when b-values reach their local minima. To examine the stability of results, two global earthquake catalogues (ISC and NEIC), different threshold magnitudes, moving-window lengths and moving steps were employed. The presented b- and p-distributions exhibit high confidence levels.
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| 18. |
- Nuannin, Paiboon, et al.
(författare)
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Variations of b-values preceding large earthquakes in the Andaman-Sumatra subduction zone
- 2012
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 61:SI, s. 237-242
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Tidskriftsartikel (refereegranskat)abstract
- Fifteen largest earthquakes, M-w >= 7, occurring between 2000 and 2010 in the Andaman-Sumatra region are studied. The available USGS list of events used is complete for a threshold magnitude M-c = 4.5 and contains 6973 shocks. Spatial and temporal variation of b-values, in the Gutenberg-Richter formula log N = a - bM, was investigated in six selected epicentral regions. The technique of moving spatial- and time-windows was applied. Deduced b-values show large variations from 0.55 to 2.40. The 15 largest events all took place within regions of low b and were preceded by significant drops in b-values. Observed correspondence between low b and the occurrence of large earthquakes suggests that b(t) has a potential which could be employed in a medium term (months, years) earthquake prediction.
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| 19. |
- Peng, Jungang, et al.
(författare)
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Triassic vegetation and climate evolution on the northern margin of Gondwana: a palynological study from Tulong, southern Xizang (Tibet), China
- 2019
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Ingår i: Journal of Asian Earth Sciences. - Amsterdam : Elsevier BV. - 1367-9120 .- 1878-5786. ; 175, s. 74-82
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Tidskriftsartikel (refereegranskat)abstract
- We present vegetation reconstructions based on an almost complete succession through the Triassic of Tulong, Nyalam County, southern Xizang (Tibet), China. The Permian and earliest Triassic samples were barren of palynomorphs, however, in overlying strata we identified well-preserved and diverse miospore assemblages. Seven pollen and spore zones spanning the Olenekian to the Rhaetian were recognized. These palynological zones were amalgamated into three floral stages that comprise distinct vegetation types: (1) an early Olenekian lycophyte dominated flora with common pteridosperms, indicative of a warm and dry climate; (2) a late Olenekian transitional flora composed of abundant conifers with low abundances of lycophytes, signifying a decrease in temperature and increase in humidity; (3) a Middle to Late Triassic mature conifer-dominated flora with diverse sphenophytes, ferns and cycadophytes, indicative of a stable, temperate and humid climate. The changes in vegetation and climate tentatively correlate with the rifting of northern Gondwana, suggesting that regional tectonics was a contributive driving factor to local floral community change.
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| 20. |
- Peng, Wenbin, et al.
(författare)
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A major change in precipitation gradient on the Chinese Loess Plateau at the Pliocene-Quaternary boundary
- 2018
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 155, s. 134-138
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Tidskriftsartikel (refereegranskat)abstract
- Spatiotemporal variations in East Asian Monsoon (EAM) precipitation during the Quaternary have been intensively studied. However, spatial variations in pre-Quaternary EAM precipitation remain largely uninvestigated, preventing a clear understanding of monsoon dynamics during a warmer climatic period. Here we compare the spatial differences in heavy mineral assemblages between Quaternary loess and pre-Quaternary Red Clay on the Chinese Loess Plateau (CLP) to analyze spatial patterns in weathering. Prior studies have revealed that unstable hornblende is the dominant (∼50%) heavy mineral in Chinese loess deposited over the past 500 ka, whereas hornblende content decreases to <10% in strata older than ∼1 Ma in the central CLP because of diagenesis. In the present study we found that hornblende is the dominant heavy mineral in 2–2.7 Ma loess on the northeastern CLP (at Jiaxian), which today receives little precipitation. Conversely, hornblende content in the upper Miocene-Pliocene Red Clay at Jiaxian is <10%, as in the central CLP. The early Quaternary abundance of hornblende at Jiaxian indicates that the current northwestward-decreasing precipitation pattern and consequent dry climate at Jiaxian must have been initiated since ∼2.7 Ma, preventing hornblende dissolution to amounts <10% as observed in the central CLP. By contrast, the 7 Ma and 3 Ma Jiaxian Red Clay hornblende content is significantly less than that of the Xifeng samples, despite the fact that today Xifeng receives more precipitation than Jiaxian, with expected enhanced hornblende weathering. This suggests that the northeastern CLP received more precipitation during the Late Miocene-Pliocene than at Xifeng, indicating that the precipitation gradient on the CLP was more east–west during the Late Miocene-Pliocene rather than northwest-southeast as it was in the Quaternary. A comparison of magnetic susceptibility records for these sections confirms this inference. We attribute this major change in climatic patterns at ∼2.7 Ma to decreased northward moisture transportation associated with Northern Hemisphere glaciation and cooling in the Quaternary. This study therefore demonstrates the potential usefulness of employing heavy mineral analysis in both paleoclimatic and paleooceanographic reconstructions.
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| 21. |
- Singsoupho, Sounthone, et al.
(författare)
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Palaeocurrent direction estimated in Mesozoic redbeds of the Khorat Plateau, Lao PDR, Indochina Block using anisotropy of magnetic susceptibility
- 2015
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 106, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- Mesozoic redbeds in the western part of central and southern Laos are the most abundant rocks of the Khorat Plateau. In earlier studies, palaeocurrent directions of the Mesozoic redbeds in the Khorat Plateau were mapped, but for the Upper Cretaceous sediments no palaeocurrent data was presented. The purpose of the present study is to test previous models of sedimentary source areas and propose palaeocurrent flow directions for Upper Cretaceous sediments in the Khorat Plateau. This we do by characterization of the rocks on the basis of their magnetic properties and anisotropy parameters. Integrated magnetic properties, including susceptibility, NRM, Q-value, and S-ratio, have been used for classification of the stratigraphies of the redbeds. A clear difference in magnetic properties was observed between the Jurassic and Cretaceous rocks, while some sites of the Lower and Upper Cretaceous were similar. From magnetic analyses the Champasak redbeds could be defined statigraphically as an unconformity beneath the Khorat Group. On the basis of the AMS interpretation, the rocks can be divided into undeformed and deformed rocks. The palaeocurrent directions in the undeformed redbeds are predominantly from the NNE and NE of the Khorat Basin. For the Upper Jurassic-Lower Cretaceous redbeds, we argue that the sources rocks of the sediments were located in the northern and eastern margin of the Khorat Plateau, probably in the Truong Son Belt zone in the northeastern Laos and central Vietnam. While, the source rock of the Upper Cretaceous redbeds in the Khorat Plateau was located in the Qinling Belt in China or in the Loei-Luang Prabang Fold Belt in northeastern Thailand and northern Laos.
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| 22. |
- Singsoupho, Sounthone, et al.
(författare)
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Tectonic Evaluation of the Indochina Block during Jurassic-Cretaceous from Palaeomagnetic Results of Mesozoic Redbeds in Central and Southern Lao PDR
- 2014
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 92, s. 18-35
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Tidskriftsartikel (refereegranskat)abstract
- Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon. = 56.0oN/178.5oE (A95 = 2.6o), 63. 3oN/170.2oE (A95 = 6.9o), and 67.0oN/180.8oE (A95 = 4.9o), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of -10.8 ± 8.8o (16.4o ± 9.0o); -11.1 ± 6.2o (17.8 ± 6.8o); and -5.3 ± 4.7o (13.3 ± 5.0o), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5 - 11o (translation of about 750 - 1700 km in the southeastward direction along the Red River Fault) and clockwise rotation of 13 - 18o with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21 - 26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.
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| 23. |
- Yuan, Qin, et al.
(författare)
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Palaeoenvironmental changes in Eocene Tibetan lake systems traced by geochemistry, sedimentology and palynofacies
- 2021
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Ingår i: Journal of Asian Earth Sciences. - Amsterdam : Elsevier. - 1367-9120 .- 1878-5786. ; 214, s. 104778-104778
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Tidskriftsartikel (refereegranskat)abstract
- Ancient lake deposits preserve detailed records of Cenozoic environmental changes, providing information on past climate, vegetation, precipitation and lake chemistry. This study focuses on palaeoenvironmental changes recorded in Eocene limnic environments across what is today the modern Tibetan Plateau. We describe a section dated as late Eocene (~38–37 Ma) and integrate these findings within a regional context of similarly-aged Tibetan lake deposits across the plateau. These sedimentary archives of environmental change indicate a period of late Eocene aridification and cooling in the lead-up to the greenhouse-icehouse transition, which remains poorly understood in Central Asia. We show, based on geochemical, sedimentological, and palynofacies analyses, that a large saline lake existed within a semi-arid to arid steppe environment in the Nangqian Basin, east-central Tibet. The saline lake experienced cyclic drying intervals with shifts to a playa lake / mudflat system. Evidence of increased aridity is recorded in the upper part of the section, including a thinning of gypsum beds, decrease in palynomorph abundance, and concurrent increase in wood debris and amorphous organic matter. This is consistent with late Eocene aridity in Asia, drying of the playa lake, and an impoverished desert-steppe vegetation. Grain size data and geochemistry indicate a stable provenance of sedimentary material, suggesting that tectonic activity did not dominate sedimentation in east-central Tibet during deposition of these successions. Rather, palaeoenvironmental changes across the Tibetan region were most probably controlled by global climate oscillations and retreat of the proto-Paratethys Sea during the late Eocene: knowledge that is relevant for ecological interpretations through the Cenozoic, Quaternary and to the present.
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| 24. |
- Zhang, Wen, et al.
(författare)
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Post-collisional Southeastern Beishan granites : Geochemistry, geochronology, Sr-Nd-Hf isotopes and their implications for tectonic evolution
- 2012
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 58, s. 51-63
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Tidskriftsartikel (refereegranskat)abstract
- Bandaoshan granites are exposed in the Southeastern Beishan area, in the central part of the Central Asia Orogenic Belt (CAOB). LA-ICP-MS U-Pb zircon dating indicates that Bandaoshan granites were emplaced into Precambrian basement at 285 +/- 4 Ma and their geochemistry indicates that they are alkali-rich potassium-high granites. Initial epsilon(Nd) values (-4.3 to -2.7) and epsilon(Hf) values (-2.7 to +0.7) suggest that Bandaoshan granites were derived from mantle-derived melt and an upper continental crustal or sedimentary component. The latter plays a significant role in their genesis. In combination with regional geology, the Early Permian Bandaoshan pluton is interpreted to form in a post-collisional environment. In the Southeastern Beishan area Late Carboniferous Qiaowan granites, Early Permian Yin'aoxia granites and Middle Permian Xijianquanzi granites are also considered as post-collisional granites, and together with Bandaoshan granites indicate that the region was in a post-collisional stage from the end of Late Carboniferous to Middle Permian.
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| 25. |
- Bercovici, Antoine, et al.
(författare)
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Permian continental paleoenvironments in Southeastern Asia: New insights from the Luang Prabang Basin (Laos)
- 2012
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120. ; 60, s. 197-211
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Tidskriftsartikel (refereegranskat)abstract
- In Laos (Southeastern Asia), Late Paleozoic sediments were identified by early French explorations across Indochina during the late 19th century (Pavie missions), but little work was undertaken to characterize the sedimentological and stratigraphical context until now. From detailed sedimentological and paleontological studies, we propose an interpretation of the depositional environment and of the stratigraphic context of series located on the right bank of the Mekong River in the Luang Prabang Basin where three main formations were described. The silicoclastic Red Claystone Formation, attributed to alluvial plain environment, contains large fragments of unidentified dicynodonts. The Limestones and Sandstones Formation preserves a new macrofloral assemblage displaying affinities with Middle to Late Permian Cathaysian floras of South China. This assemblage occurs as an intercalation within marine calcareous sandstones that have yielded a marine fauna, including the ammonoid Pseudotirolites sp. which indicates a Late Permian (Changhsingian) age. The well-developed Purple Claystones Formation yielded an abundant and well preserved Late Permian fauna composed of a carnivorous amphibian and numerous Dicynodon cranial and postcranial elements. This formation shows a vertical evolution from braided river to alluvial plain with sheet-flood sand bed and bed-load rivers, with a constant supply of volcanic clasts. Results from the analysis of the paleontological associations in the Luang Prabang Basin suggest that a continental communication between Laurussia and the Indochina Block existed during the Permian, allowing for migration of the terrestrial Dicynodon fauna. (C) 2012 Elsevier Ltd. All rights reserved.
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