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1. chinaXiv:202112.00019 [pdf]

Syn-subduction intra-continental shearing during Oligocene in Indochina, SE Asia

Zhang, Ji'en; Xiao, Wenjiao; Zhang, Bo; Wakabayashi, John; Cai, Fulong; Sein, Kyaing
Subjects: Geosciences >> Geology

Indochina developed continental-scale shear zones that record Cenozoic tectonic processes in SE Asia. Previous extrusion models that linked these shear zones with the northward indentation of the Indian continent, conflict with distributed conjugate strike-slip pairs and post-Oligocene rotation in Indochina. This paper presents evidence of coeval shearing along the Mogok-Shan Scarp fault in Myanmar, the western boundary of the Indochina block, previously proposed as a product of northward indentation of Indian Plate. The Kyanigan quarry along the northern Mogok-Shan Scarp fault exposes paragneiss, marble and quartzite schist. ‘σ’ structures, cored with garnet, in paragneiss record right-lateral shear sense, consistent with ‘σ’ and ’δ’ structures in L-tectonites in the Moulmein granitic mylonite to the south. U-Pb ages of metamorphic zircons of paragneiss and a biotite 39Ar-40Ar age of quartzite schist constrain deformation in the Kyanigan quarry to 33-21 Ma; a biotite 39Ar-40Ar age of mylonite at Moulmein is 26 Ma. These ages demonstrate Oligocene right-lateral shearing along the Mogok-Shan Scarp fault, coeval with other shear zones in Indochina. These Oligocene shear zones and strike-slip faults are conjugate structures recording left-lateral shear sense on NW-striking and right-lateral shear sense on N-S to NE-striking features. After restoration of ~80° clockwise rotation, the Oligocene conjugate strike-slip pairs in Indochina reflect approximate N-S-directed shortening, corresponding to northward subduction of the Indian ocean. This suggests that continental-scale intra-continental shearing may have been triggered by syn-subduction compression in SE Asia.

submitted time 2021-12-01 Hits87Downloads45 Comment 0

2. chinaXiv:202110.00042 [pdf]

陕西横山 L2 以来风沙/黄土沉积序列的粒度端元 特征及其环境意义

刘蓉
Subjects: Geosciences >> Geology

受气候振荡的影响,沙漠边界多次往复摆动,在地层中留下了黄土、古土壤与古风成沙的迭 覆沉积。通过传统粒度参数和端元分析模型方法对陕西横山 HS 剖面 L2 以来风成沙—沙质黄土— 沙质古土壤所蕴含的粒度成分、相应的沉积信息和沙地进退进行探讨。结果表明:HS 剖面粒度组 成不同于黄土区,以极细砂(31.07%)、细砂(30.20%)和粗粉砂(23.38%)为主,具有大小混杂的宽粒 级范围的明显特征。因此在黄土区具有全球古环境意义的粒度指标未必适合本区,运用参数化端 元分析模型对本区粒度指标进行分离,以期得到适宜本区的气候环境指标。其中,端元 1(EM1)的 众数粒径为 8.93 μm,反映了西风环流的信息;端元 2(EM2)的平均粒径为 32.82 μm,很大程度上间 接指示东亚冬季风强度的变化;端元 5(EM5)的平均粒径为 235.46 μm,是极强冬季风或强风暴的 替代性指标,其含量反馈当时冬季风的强烈程度。由此,研究认为陕西横山自 L2 以来发生了 5 次沙 地扩张和 3 次沙地后退事件,其中风成沙层为强烈冬季风环境引发毛乌素沙地扩张时堆积的;而冬 季风强度大为减弱时,沙丘迁移被沙尘堆积所取代,形成沙质黄土层;古土壤层是在冬季风相对萎 缩时发育的。冰期形成的风成沙层对目前区域沙漠化存在不可忽视的潜在威胁,保护全新世形成 的土壤层是防治区域沙漠化的重要措施。

submitted time 2021-10-10 From cooperative journals:《干旱区地理》 Hits82Downloads51 Comment 0

3. chinaXiv:202110.00044 [pdf]

新疆罗布泊地区白龙堆雅丹地貌形态特征及成因研究

林桂权; 林永崇
Subjects: Geosciences >> Geology

雅丹地貌是风沙地貌研究的薄弱环节,目前从形态描述到定量研究还未出现较大突破,亟 待结合新的技术手段开展创新性研究。新疆罗布泊地区极端干旱,风蚀作用强烈,广泛分布着不 同时期发育的雅丹,是研究雅丹地貌形态的理想区域。在 Google Earth 上沿着新疆罗布泊地区盛行 风向测量并分析了白龙堆雅丹 4 个子区域共 1000 个雅丹个体的长度、宽度等形态参数,结果表明: 白龙堆雅丹 4 个子区域长度、宽度和长宽比(R 值)等形态参数存在一定区域差异,但总体上相近, 其雅丹长度均值为 63.42 m,65%集中在25.00~75.00 m 之间;宽度均值为 13.97 m,80%分布在 5.00~ 20.00 m 之间;R 值均值为 4.37,80%集中在 2.00~6.00 之间。白龙堆雅丹 R 值的均值总体上接近于 特定风场条件下发育的雅丹 R 值(R=4.00)。因此,区域风蚀作用塑造了相对稳定的雅丹形态,并且 这种形态基本接近于特定条件下风蚀形成的雅丹形态。尽管风动力是白龙堆雅丹形态主要的塑 造动力,但是地层岩性、地表盐壳、极端干旱气候等因素也具有重要的影响。白龙堆雅丹形态变化 特征可为深入理解雅丹地貌发育过程提供区域性认识。

submitted time 2021-10-10 From cooperative journals:《干旱区地理》 Hits112Downloads65 Comment 0

4. chinaXiv:202108.00077 [pdf]

玛曲高寒草甸风沙环境与沙化类型研究

安志山
Subjects: Geosciences >> Geology

玛曲县高寒草甸沙化是青藏高原地区发生沙化的典型案例,其成因、趋势和治理一直受到极大关注。利用三维激光扫描仪,结合野外监测和室内分析等方法,揭示了区域风沙环境,结合区域沉积物粒度与形态特征,依据沙物质来源复杂程度将沙化类型划分为风蚀坑沙化类型和复杂沙化类型。结果表明:区域全年输沙势为164.34VU,合成输沙势为91.57 VU,合成输沙方向132.37°,风向变率0.56,属于中比率低风能环境。综合风蚀坑沙化类型周边风蚀坑与积沙区的长度比、体积比、长轴线、与区域主风向吻合程度及沉积物粒度特征,可知风蚀坑是该沙化类型区域积沙的主要贡献者。在现代沙化过程中,复杂沙化类型除周边风蚀区、活化沙丘以及沙化草地等为区域积沙贡献沙物质外,黄河河道也为区域积沙提供一定量的沙物质。

submitted time 2021-08-12 From cooperative journals:《干旱区研究》 Hits176Downloads112 Comment 0

5. chinaXiv:202106.00106 [pdf]

A fragment of Argoland from East Gondwana in the NE Himalaya

Ji’en Zhang; Wenjiao Xiao; John Wakabayashi; Brian F Windley; Chunming Han
Subjects: Geosciences >> Geology

Previous studies have concluded that a Trans-Tethyan oceanic subduction zone existed prior collision of India-Eurasian plates, between which the ocean lacked intervening continental slivers. In contrast, we present first geological evidence of Early Cretaceous shortening and Late Jurassic alkali magmatism constraining that the Longzi block, an extensive (>450 km E-W by ca. 130 km N-S) tract of the NE Himalaya is such a continental sliver. The Longzi block records overturned south-vergent folds in Triassic to Lower Cretaceous strata intruded by 136-123 Ma mafic, dioritic, and dacite dikes, constraining Early Cretaceous shortening. The shortening demonstrates the NE Himalayan locating in a compressional setting, rather than an extensional Indian passive continental margin at that time. Triassic strata of NW Australian affinity and Late Jurassic rocks sourced from north India record pre-rifting history. Rifting evidence includes 152.8 Ma alkali intrusive rocks, a Late Jurassic unconformity, and rapid changes in sediment thickness and apparent water depth of deposition recorded in Upper Jurassic strata. The rifting event is coeval with 152-155 Ma oceanic crust in the NE Indian Ocean and a Late Jurassic submarine escarpment with 1200 m of sediments offshore of NW Australia. These data may reflect rifting of the westernmost Argoland continent in NE Himalaya from East Gondwana, followed by collision with a N-dipping Trans-Tethyan intra-oceanic subduction zone in the Early Cretaceous, long before terminal continent-continent collision. The Mesozoic rifting-collision in the Himalayan region unambiguously presents archipelagic paleogeography in eastern Neotethyan, which underwent Cenozoic two-stage Indian-Eurasian collisional processes.

submitted time 2021-06-24 Hits4048Downloads424 Comment 0

6. chinaXiv:201907.00003 [pdf]

Sub-parallel ridge-trench interaction and an alternative model for the Silurian-Devonian archipelago in Western Junggar and North-Central Tianshan in NW China

Zhang, Ji'en; Chen, Yichao; Xiao, Wenjiao; Wakabayashi, John; Windley, Brian F.; Yin, Jiyuan
Subjects: Geosciences >> Geology

Plate boundary evolution and interpreted paleogeography commonly involve the interaction between oceanic ridges and trenches. Western Junggar and Central-North Tianshan, NW China, have previously been regarded as independent orogens, but this model was challenged by the discovery of their similar tropic-subtropic coral assemblages and of Precambrian and Early Paleozoic clasts transmitted from the Central Tianshan in Early to Middle Paleozoic sediments in SW Junggar, suggesting they had been in contact by then. The presence of Late Silurian-Early Devonian adakitic rocks in northern SW Junggar was explained by slab roll-back, which required an active subducting slab in SW Junggar. However, this model is inconsistent with the cessation of arc magmatism immediately after the adakitic magmatism indicating there was no subduction in SW Junggar at those times. By re-evaluating the relevance of four diagnostic features of modern Californian ridge subduction, we propose a new model to explain the Late Silurian-Early Devonian orogenic framework of NW China in terms of sub-parallel ridge-trench interaction. The transmitted clasts from Central Tianshan in Early Paleozoic sediments in SW Junggar, at the same time as the Late Ordovician and Middle Silurian coral assemblages, and the complementary accretionary complex in SW Junggar and magmatic arc in Central Tianshan, are all contributary evidence for an Early Paleozoic subduction system. This subduction zone was intruded by two 446-380 Ma double magmatic belts: 1. A ridge subduction-generated adakitic gabbro-basalt-andesite-diorite-granite-rhyolite suite that intruded a 504-446 Ma accretionary complex in SW Junggar, and 2. a magmatic arc in Central Tianshan. A buoyant subducting ridge rifted and separated these magmatic belts resulting in the opening of a new 414-325 Ma ocean, which is preserved in OIB- and MORB-type ophiolites in SW Junggar and North Tianshan. These geological and geochemical relationships resemble those generated by oceanic spreading in the modern Gulf of California, which led to separation of magmatic belts in Baja California and mainland Mexico. The new ocean split the Central Tianshan magmatic arc from the Early Paleozoic Mt. Xiemisitai-Barleik-Mayile accretionary complex in SW Junggar, which changed to a new passive continental margin that led to deposition of Devonian shallow marine-terrestrial sediments, and to cessation of magmatism at 379-349 Ma. These features, including the Middle Paleozoic orogenic architecture and the geochemical properties of specific magmatic rocks, indicate the development of a Late Silurian-Early Devonian sub-parallel-to-trench ridge subduction. This model not only interprets phenomena mentioned in the above earlier models, but also reconciles unresolved spatial relationships between Western Junggar and the Tianshan. Subsequent closure of the new ocean and subduction of its ridge in the Late Paleozoic gave rise to the archipelagic framework that we see today in NW China. An analysis of worldwide examples of sub-parallel ridge-trench interaction demonstrates that a ridge can undergo multiple episodes of subduction before a terminal ocean closure. In this context our new tectonic model for the western Junggar and Tianshan regions may provide an improved understanding of the structure and evolution of ridge-subduction events in ancient orogenic belts.

submitted time 2021-06-22 Hits7570Downloads1451 Comment 0

7. chinaXiv:202104.00032 [pdf]

延安地区丹霞地貌类型及发育机制研究

彭小华
Subjects: Geosciences >> Geology

延安地区丹霞地貌是国内外地貌学界近年来的最新发现和持续关注点。通过对延安地区 丹霞地貌地质遗迹的系统调查和研究发现,该区丹霞地貌景观资源丰富,类型多样,具有较高的观 赏价值和科学研究价值。通过野外考察和岩石样品分析,从地质构造、地层组合、岩性特征和外动 力等方面来探讨延安地区丹霞地貌的发育机制。研究表明:晚白垩世以来,该地区经历多次构造 抬升和新生代以来地壳保持差异升降,区内构造简单,地层产状平缓,但节理、裂隙数量较多,对延 安地区丹霞地貌的发育具有控制作用;延安地区丹霞地貌造景岩层主要为白垩系志丹群洛河组(K1l),为一套干旱气候条件下的红色碎屑岩沉积,沉积环境为沙漠环境,岩层倾角近于水平,为丹 霞地貌的发育提供了较好的物质基础;岩性特征上,延安地区丹霞地貌造景岩层岩石成熟度不高,硬度低易碎,抗侵蚀能力较弱,岩石具有相似的矿物组成及峰值强度分布,其沉积过程经历了较为 复杂的风力作用过程,岩石强度存在差异性,易受到后期外动力作用的改造;外动力方面,季节性 的流水是塑造“沟谷型”丹霞地貌的主要外动力,此外,凹片状和凸片状风化作用以及崩塌作用对 延安丹霞地貌的发育也有重要影响。

submitted time 2021-04-13 From cooperative journals:《干旱区地理》 Hits679Downloads405 Comment 0

8. chinaXiv:202006.00209 [pdf]

柴达木盆地昆北地区路乐河组/下干柴沟组 泥岩地层地球化学特征及古环境意义

余平辉; 马锦龙
Subjects: Geosciences >> Geology

柴达木盆地昆北地区位于柴达木盆地西部南区的昆仑山前缘,该地区构造活动频繁,地层完整,是研究古环境和古气候理想地区。本文采集了昆北地区切26井新生界下部泥岩样品,并对其进行了微量元素分析。研究表明:在地层界线附近,路乐河组Cu、U、Cr、Sr元素含量平均值分别为18.34、3.24、141.68、107.10 ppm,下干柴沟组下段为28.17、3.35、192.07、217.05 ppm。下干柴沟组下段微量元素有明显增大趋势,指示沉积环境发生变化。U与陆源碎屑来源指示元素Th相关性差(R2=0.174 5),表明U主要为自生成因,因此其含量的增加可以指示原始沉积水体还原程度的增强。Cr与U表现为弱相关(R2=0.42),表明水体环境的变化对Cr的富集有一定的影响;氧化还原指标U/Th、V/Cr表明研究区古环境为氧化水体沉积环境,U/Th、V/Cr指标在下干柴沟组下段变大表明水体加深,还原性增强。利用Sr/Cu比值变化以及结合化石种属演化等推测地层界线附近古气候经历了干旱-湿润-干旱的旋回变化。

submitted time 2020-06-12 From cooperative journals:《干旱区地理》 Hits1215Downloads772 Comment 0

9. chinaXiv:202003.00081 [pdf]

滑面粘土FIB-SEM分析

赵宇; 胡良博; 蒋宇; 唐俊峰; 黄栋; 薛华庆; 周尚文
Subjects: Geosciences >> Geology

本文通过原位分析计算,获得滑面、邻近面特征粘土矿物种类及其相对含量、矿物粒径及孔隙率,实验以清晰可靠的物理图像和测试数据揭示滑面、邻近面矿物成分、结构的空间分布特征和差异,从而揭示土质斜坡破坏、滑面强度降低的物理路径和机制。实验采取新鲜未经后期风化滑带土,用AMICSCAN矿物分析电镜集高分辨场发射扫描电镜、最新一代(第三代)的矿物自动分析软件AMICS(Automatic Mineral Identification and Characterization System,分辨率可达0.5μm像素)和超大面积高分辨成像软件为一体的矿物分析和结构分析系统,依据矿物原子比例的唯一性,在识别矿物种类及其分布情况。其次,利用 FEI 公司的Helios NanoLab-650 聚焦电子束扫描电镜(FIB-SEM)并结合X 射线能谱仪(EDS)/以电镜的高分辨率背散射电子二维图像与能谱仪测得的特征谱相结合,借助Avizo Fire 软件基于能谱的点、线元素扫描图像和每间隔一微米扫描得出500帧聚焦电子束扫描电镜图像结合,可视化重建并计算滑带土的孔隙空间三维分布状态,并通过体积比近似折算成含量比,为揭示滑面形成路径和机理提供真实、准确可靠的证据。实验显示:1)微米尺度上,滑面、邻近面之间可见明显的分界,滑面厚度不均匀,在10纳米分辨率的背散射图像上,滑面结构致密孔隙小且较难分辨;2) 宽17微米,高19微米的立方体扫描切割3维立体滑带土样中,滑面部分孔隙率0.0331,邻近面部分孔隙率0.0754,邻近面孔隙率约为滑面的2.3倍;3)能谱仪特征谱图像和数据显示,滑面上蒙脱石特征元素Na、Ca、Mg峰明显,并且在分界线附近出现跳水式的陡降,证明滑面上蒙脱石富集,而邻近面却未检出蒙脱石;4)邻近面伊利石特征元素K高于滑面,即邻近面伊利石含量明显高于滑面。实验用清晰的物理图像和准确的能谱数据证实:滑面、邻近面存在矿物成分和结构的显著差异,滑面孔隙率低有可能是因为滑面上覆土体压力造成的。本文首次以原位实验的方法揭示滑面、邻近面矿物成分、孔隙率和结晶度及粒度空间差异,为揭示滑面演化路径提供了可靠的依据,为最终建立滑面、土体跨尺度演化模型,我们需要进行更多的滑面、邻近面成分结构差异、宏观强度、微纳尺度结构对比分析、以及矿物演化动力学过程分析。

submitted time 2020-03-26 Hits9678Downloads1996 Comment 0

10. chinaXiv:202003.00082 [pdf]

Measuring colloidal forces between clay microparticles with optical tweezers

赵宇; 武京治; 胡良博; 蒋宇
Subjects: Geosciences >> Geology

The interaction forces between clay micro-particles play an important role in the macroscopic strength behavior of clayey soils. Optical tweezers were used in the present study to explore the interaction between clay micro- particles. This technology uses a highly focused laser beam to manipulate small objects and can also be used as a force transducer for the measurement of forces on the order of pico-Newtons (pN). Polystyrene beads were first used to measure the surface interactions between polystyrene beads and clay particles for accurate calibration of the system because of their perfectly spherical shape and optical homogeneity, and were successful in obtaining force mea- surements within the range of 20 pN. Subsequently the interactive force was measured when a small clay particle was moved along the surface of a large clay particle. The force measured varies as the interaction of clay surfaces may evolve along their relative motion, leading to force measurements up to 40 ? 80 pN. The present study shows a promising potential of optical tweezers in exploring the complex micro-scale phenomena in clay minerals.

submitted time 2020-03-26 Hits9495Downloads1876 Comment 0

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