Rapid tectonics of the Late Miocene Boso accretionary prism related to the Izu–Bonin arc collision

Abstract   The structure, paleomagnetism and biostratigraphy of the Nishizaki and Kagamigaura formations on the southern Boso Peninsula, central Japan, were investigated to determine the chronographic constraints on the accretion, post-Late Miocene rotation and regional tectonics in the Izu–Bonin island arc collision zone. The geological structures on the southern Boso Peninsula are characterized by an east–west trending and south-verging fold and thrust belt that curves toward the northwest–southeast in the northwest extent of the Nishizaki Formation. Two stages of tectonic rotation were revealed by paleomagnetic and structural studies. The first is believed to have occurred after the accretion of the Nishizaki Formation and before the deposition of the Kagamigaura Formation, while the second is confidently correlated with the 1 Ma Izu block collision. The northwest extent of the Nishizaki Formation was rotated clockwise by approximately 65–80°, whereas the rotation was only 25–30° in the east, and 11–13° in the overlying Kagamigaura Formation. Radiolarian biostratigraphy suggests a depositional age of 9.9–6.8 Ma (Upper Miocene period) for the Nishizaki Formation and 4.19-3.75 Ma (Pliocene period) for the lower Kagamigaura Formation. These results indicate that the age of accretion and first-stage rotation of the Nishizaki Formation can be constrained to the interval of 6.80–3.75 Ma. This structure most likely represents the northward bending caused by collisions of the Tanzawa and Izu blocks with the Honshu island arc, and suggests rapid processes of accretion, collision, uplift and the formation of new sedimentary basins within a relatively short period of time (2.61–3.05 my).     

雪峰山隧道局部涌水模型探讨

雪峰山隧道施工后，对地下水进行了实时监测，并分析了雪峰山隧道涌水的基本情况，推导出三棱柱涌水模型，并加以验证，说明该模型的实用价值。     

Field report: Sailing around the exhumed roots of the Mesozoic Patagonian paleo-accretionary wedge(Diego de Almagro Island,Chile)

We undertook a boat expedition to explore the geological framework of a very remote, lesser-known island, in the Chilean Patagonia: the Diego de Almagro Island(latitude S51°330'). This uninhabited, ca.400 km~2 Island is one of the very rare exposures of the Mesozoic accretionary subduction complex along the Chilean margin. Unstable weather, strong winds, steep topography, and very dense vegetation make an on-land mission difficult. Careful preparation based on high-resolution satellite images is advised to optimize shore access and minimize risks of injury. Despite a relatively important degree of regional reequilibration of metamorphic assemblages due to sluggish exhumation through the forearc crust, our results have shown that the island is composed of a nappe stack of ocean-floor derived slivers of metasedimentary units that exhibit very different pressure-temperature-time paths during burial by subduction under the Chilean margin and subsequent exhumation. These rocks are witness to a complex thermal evolution of the subduction zone between Jurassic and Cretaceous times from granulite facies to blueschist facies conditions as well as multiple episodes of accretion at ca. 35 -40 km in depth for almost100 Ma over the Mesozoic era.     

北秦岭造山带早古生代多期变质与深熔作用：锆石U-Pb年代学证据

北秦岭造山带中的秦岭群以多期变质作用和强烈混合岩化为特征,出露有早古生代的榴辉岩和高压麻粒岩,以及区域性分布的中低压麻粒岩-角闪岩相变质岩石和混合岩。本文对西峡双龙地区混合岩化片麻岩(中色体)、石榴子石黑云母片岩(暗色体)和淡色花岗岩脉体进行了详细的锆石U-Pb定年和微量元素研究。结果表明片麻岩的原岩年龄为941±11Ma(2σ,MSWD=0.59),石榴子石黑云母片岩的原岩年龄为756±9.9Ma(2σ,MSWD=1.07)。石榴子石黑云母片岩中的变质锆石获得了484±9.6Ma(2σ,MSWD=0.88)的年龄,与片麻岩中锆石变质增生边获得的单点年龄498±11Ma在误差范围内一致。这些变质锆石多数具有平坦的HREE分配模式,弱Eu负异常,与高压变质岩中的变质锆石特征相似。该年龄与秦岭榴辉岩和高压麻粒岩的变质年龄相一致。石榴子石黑云母片岩中锆石的最外层增生边记录了424±9.1Ma的年龄,同时淡色脉体中的锆石也给出了422±4.0Ma(2σ,MSWD=0.77)的加权平均年龄,代表了深熔脉体的结晶年龄。这些锆石均为新生锆石,阴极发光弱,具有低的Th/U比值,平坦的HREE分配模式,强烈的Eu负异常,指示部分熔融过程中存在大量石榴子石和斜长石。深熔脉体的年龄与秦岭群中的中低压高温-超高麻粒岩相变质的年龄相符。结合已有的结果,我们认为秦岭群普遍记录了500~480Ma的高压、超高压变质作用,并叠加了440~400Ma的中压-高温变质和深熔作用。秦岭群可能是Rodinia超大陆裂解过程中从华南陆块或相似构造属性陆块分离并漂移到华北克拉通南缘的微陆块。在500~480Ma时发生碰撞造山作用导致北秦岭微陆块深俯冲并发生了高压-超高压变质作用,在志留纪由于商丹洋壳北向俯冲导致秦岭微陆块发生了以中压-高温变质、深熔和同时的岩浆作用为特征的增生造山作用。     

Misattributed tsunami 3: the Mw 7.7 2013.9.24 SE Pakistan earthquake

The M_w 7.7 earthquake that struck SE Pakistan on 24 September 2013 at 11.29.48 UTC was a sinistral strike-slip event on a branch of the Ornach-Nal-Chaman fault system which hereabouts separates the Eurasian Plate from the Indian Plate. Although the focus was at a depth of 15 km and 400 km inland the earthquake was accompanied by the emergence of an island off the Makran coast and the generation of a tsunami with a peak amplitude of 27 cm at Muscat (Oman) and 20 cm at Chah Bahar (Iran). At DART marine buoy 23228 in the Indian Ocean 500 km to the south a series of seismic Rayleigh waves about 4 min after the main shock was followed 54 min later by a tsunami with a peak amplitude of 1 cm. The Rayleigh series is here attributed to seafloor vibration during accelerated subduction of the Arabian Plate beneath the Eurasian Plate, and the tsunami to the development or reactivation of one or more reverse faults on the seaward portion of the Makran imbricate fan. As in the 2010.2.27 M_w 8.8 Maule (Chile), the 2004.12.26 M_w 9.2 Sumatra–Andaman, the 2005.3.28 M_w 8.7 Nias (Indonesia) and the 2011.3.11 M_w 9.0 Tohoku (Japan) earthquakes, the link between tsunami generation and slip on the megathrust is thus very indirect, to the detriment of attempts to mitigate coastal hazards using teleseismic data when nearshore seafloor monitoring would probably prove more effective.     

新疆蛇绿岩时空分布特征及对增生造山过程的制约

聚焦新疆区内集中出露的61处蛇绿岩,据其物质组成、构造属性、形成时代、空间分布等特征,将其划分为14条蛇绿混杂岩带,其中多处发育洋岛海山、洋内弧等大洋岩石圈岩石组合,并以塔里木-敦煌地块为界,提出以北属古亚洲洋构造域、以南属特提斯洋构造域.结合俯冲增生造山过程中不同阶段的岩石学记录,确认古亚洲洋形成于新元古代末期至晚石...     

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing ’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin-Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260 –240 Ma), respectively, forming the Xinlin-Xiguitu, Mudanjiang-Yilan, Hegenshan-Heihe, Solonker-Linxi, and Changchun-Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary orogenic belts in central Asia.©2022 China Geology Editorial Office.     

基于潜在活动空间的个体时空可达性特征与影响因素研究——以上海市郊区为例

可达性是人文地理学和相关学科研究的核心议题之一。在“以人为本”理念的影响下,以时间地理学理论为基础、基于人的研究范式的时空可达性测度方法受到学者关注,成为生活质量、社会公平等议题的重要切入点。论文通过刻画整日潜在活动空间对个体的时空可达性进行测度,并以上海市郊区为案例地区,基于2017年居民活动日志一手调查数据开展实证研究。首先利用路网分析、二次开发等方法,对个体工作日和休息日的潜在活动空间进行测度;其次以弹性时间、整日潜在活动空间面积和可达设施密度为测度指标,利用GIS三维可视化、方差分析等方法分析时空可达性的特征及其在空间和时间维度的分异;最后,利用多元回归分析方法,探讨区位因素、时间因素和社会经济属性对居民时空可达性的影响。研究结果表明,上海市郊区居民的时空可达性在空间和时间维度上均存在着明显的分异,其中远郊居民面临着更强的时空制约和更大的空间困境;区位因素和时间因素是影响居民时空可达性的重要因素。该研究是时空可达性的测度方法在郊区中的实证检验,揭示了时空可达性的动态特征和个体间差异,为设施的时空优化配置和郊区新城建设中居民生活质量的提高提供了实证依据。     

Formation of the Yakuno ophiolite; accretionary subduction under medium-pressure-type metamorphic conditions

The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during the Permian evolution of Japan on the Yakuno eruptive sequence, association of hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening of sequences terrigenous sandstone and conglomerate, and mildly deformed Permian and Triassic forearc basin formations. The most important indicator, however, is the seaward imbrication and repetition observed in both the Maizuru Group and the ophiolite itself. D1 deformation structures include axial–planar foliations (pressure-solution cleavage for the Maizuru Group and granulite–amphibolite metamorphic layering in the ophiolite), flattening type strain, symmetric pressure shadows and fringes, and isoclinal folds showing axial–planar foliations and thrust faulting at their overturned limb. The exceptional asymmetry observed indicates seaward-directed shearing near the thrust, while D1 structures in the Maizuru zone are explained by off-scraping, above the basal decollement. The later Jurassic D2 kink fold structure includes a first-order asymmetric kink with a brittle thrust at its overturned limb, more-or-less coeval with M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism in the Yakuno ophiolite produced layering of granulite and amphibolite, and in the Maizuru Group, formation of illite along pressure-solution cleavage of mudstones. The metamorphic grade is controlled by the stratigraphic relationships and appears typical of that in ocean floor regions. However, there was only one episode of M1 prograde metamorphism which occurred contemporaneously with D1 off-scraping. Given that subduction zones are normally characterized by high P/T metamorphic regimes, the observed P/T history appears to reflect relatively unusual conditions. Such high thermal gradients may plausibly reflect the approach of a young, hot oceanic plate which continued subducting beneath the Japanese arc. Accordingly, the Yakuno ophiolite was probably formed at the trench–trench–ridge triple junction.     

阿拉伯海北部莫克兰增生楔天然气水合物浅表层识别标志

阿拉伯海北部的莫克兰增生楔是阿拉伯板块以低速、低角度俯冲到欧亚板块之下形成的主动陆缘构造,蕴藏着丰富的天然气水合物资源。依据2019年中国在莫克兰增生楔海域采集的高分辨率多道地震资料、浅地层剖面及多波束测深数据,并结合以往的调查成果,探讨莫克兰海域天然气水合物存在的浅表层识别标志。地震识别标志主要有似海底反射层(BSR)和振幅空白带2种标志,地形地貌标志包括海底麻坑、海底滑塌、丘状体、泥火山、冷泉系统等,水体标志主要为羽状流。在水深1000m和2900m的站位已分别钻获水合物样品。莫克兰增生楔丰富的水合物识别标志可能与低速、低角度的俯冲地质背景有关,使该区水合物存在指示兼具主动大陆边缘和被动大陆边缘的特征。综合研究区的异常标志分布特征,推测增生楔中部和西部的背斜脊及其附近区域是天然气水合物远景区。