New Silurian and Devonian palaeomagnetic results from the Hexi Corridor terrane, northwest China, and their tectonic implications

A total of 239 orientated drill-core samples from 23 sites were collected for palaeomagnetic study from Silurian and Devonian red beds, marlaceous sandstone, and limestone rocks in the eastern part of the Hexi Corridor, southwest Ningxia, North China. The characteristic high-temperature component resides in both haematite and magnetite. It clusters around a northwesterly and shallow to moderate downward direction and its antipode after tilt correction. The primary origin of this characteristic remanent magnetization (ChRM) is ascertained by positive fold and reversal tests at the 95 per cent confidence level. The corresponding palaeopoles, at 339.0°E, 60.1°N with A ₉₅ = 11.2° (Silurian) and 336.0°E, 56.0°N with A ₉₅ = 9.2° (Devonian), imply that the North China Block (NCB) had a low palaeolatitude of around 15°N in the Northern Hemisphere during the Silurian–Devonian period. Comparison with the Early–Middle Ordovician palaeopole of the NCB suggests that the NCB moved rapidly northwards by 30.8° ± 10.9° to cross the palaeo-equator during the Early–Middle Ordovician to Silurian. In combination with the palaeobiogeographical data from Ningxia, our palaeomagnetic results suggest that the NCB was located close to Australia during the Late Devonian.     

华北陆台基底构造格局研究现状一瞥

专家们对华北陆台早前寒武纪基底构造格局形成与演化的认识既有共同点又有分歧。共同点是太古宙为形成刚性小陆块的时期 ,其成因可能与地幔柱的垂直增生有关 ,如TTG质岩类的大量增生与侵位。小陆块的构造拼合形成华北克拉通的主体。分歧焦点是陆块拼合的时代 :第一种认为小陆块的拼合发生在新太古代 ,即华北陆台在新太古代就已克拉通化 ,到古元古代时在伸展构造体制作用下 ,形成了一系列的裂谷或拗拉谷。第二种认为拼合发生在古元古代 ,即华北陆台是吕梁运动才克拉通化的。形成裂谷或拗拉谷的时期在中元古代     

Controls on the evolution and demise of Lower Carboniferous carbonate platforms,northern margin of the Dublin Basin,Ireland

Shallow water platform limestones of the Chadian–Asbian Milverton Group are restricted to the north-eastern part of the Lower Carboniferous (Dinantian) Dublin Basin. Here, they are confined to two granite-cored fault blocks, the Kentstown and Balbriggan Blocks, known to have been active during the late Dinantian. Three areas of platform sedimentation are delimited (the Kentstown, Drogheda and Milverton areas), although in reality they probably formed part of a single carbonate platform. Resedimented submarine breccias and calciturbidites (Fingal Group) composed of shallow water allochems and intraclasts sourced from the platform accumulated, along with terrigenous muds, in the surrounding basinal areas. Sedimentological evidence suggests that the Kentstown and Balbriggan Blocks possessed tilt-block geometries and developed during an episode of basin-wide extensional faulting in late Chadian time. Rotation of the blocks during extension resulted in the erosion of previously deposited sequences in footwall areas and concomitant drowning of distal hangingwall sequences. Antithetic faults on the northern part of the Balbriggan Block aided the preferential subsidence of the Drogheda area and accounts for the anomously thick sequence of late Chadian platform sediments present there. Continued subsidence and/or sea-level rise in the late Chadian–early Arundian resulted in transgression of the Kentstown and Balbriggan Blocks; carbonate ramps developed on the hangingwall dip slopes and transgressed southward with time. Subsequent progradation and aggradation of shallow water sediments throughout the Arundian to Asbian led to the development of carbonate shelves. Several coarse conglomeratic intervals within the contemporaneous basinal sequences of the Fingal Group attest to periodic increases of sediment influx associated with the development of the shelves. Sedimentological processes controlled the development of the carbonate platforms on the hangingwall dip slopes of the Kentstown and Balbriggan Blocks, though periodic increases of sediment flux into the basinal areas may have been triggered by eustatic falls in sea level. In contrast, differential subsidence along the bounding faults of these blocks exerted a strong control on the margins of the late Dinantian shelves, maintaining relatively steep slopes and inhibiting the progradation of the shelves into the adjacent basins. Tectonically induced collapse and retreat of the platform margins occurred in the late Asbian–early Brigantian. Platform sediments are overlain by coarse-grained proximal basinal facies which fine upwards before passing into a thick shale sequence, indicating that by the late Brigantian carbonate production had almost stopped as the platforms were drowned.     

A comparison of rainfall estimation techniques for sub-Saharan Africa

Interpolated rain-gauge data were compared to Meteosat-based precipitation estimates for sub-Saharan Africa. Validation was carried out using a dataset from a very dense gauge network in South Africa, on a point-to-pixel (PO–PI) as well as on a pixel-to-pixel (PI–PI) basis. Error criteria computed at the gauged pixels indicate that overall the interpolated estimates perform similarly to the satellite-based data: they provide good estimates of lower but underestimate larger precipitation amounts. It is concluded that the satellite estimates are more fitted for the operational modelling of processes such as surface runoff and soil erosion, especially in the developing areas where resources are scarce.     

Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block,China

The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some ...     

Analysis of ecological quality in Lhasa Metropolitan Area during 1990–2017 based on remote sensing and Google Earth Engine platform

Based on a total of 519 images,the composite images with the lowest possible cloud cover were generated at pixel level with image synthesis method on Google Earth En-gine(GEE)platform.The Remote Sensing Ecological Index(RSEI)was adopted,and calcu-lated in an efficient way with the assistance of parallel cloud computing of the GEE platform.The RSEI was used in this paper to evaluate and monitor the eco-environmental quality of the Lhasa Metropolitan Area.Results show that:(1)The ecological quality is better in the west than in the east of Lhasa Metropolitan Area,with Lhasa as an approximate dividing point.The ecological quality improved and then deteriorated dramatically before 2000,with the mean RSEI value dropping from 0.51 to 0.46;the trend was followed by a gradual increase up until 2017,with the mean RSEI value increased from 0.46 to 0.55.(2)The RSEI is weakly and positively correlated with socioeconomic indicators.This indicates that the population growth and economic development did not negatively influence the ecological quality,but actually boosted it.(3)The GEE can serve as an efficient computing platform for the assessment and monitoring of eco-environmental quality in vast regions.     

华南陆块岩石圈有效弹性厚度及其构造意义

华南陆块受多阶段超大陆聚合、裂解,碰撞、陆内造山,及伸展等作用影响,造成其深部结构和构造极其复杂。岩石圈有效弹性厚度(Te)是表征地质时间尺度上岩石圈力学强度的定量指标,可为深入认识岩石圈力学结构及演化提供有效约束。本文基于导纳和相关函数联合方法对地壳布格重力异常和地形数据进行计算,获得华南陆块Te的空间分布。Te高值(>20 km)区域主要分布于扬子地块的四川盆地周边区域,而Te低值(<20 km)区域集中于华夏地块和江南造山带区域。由于Te分布特征与地热场、地震关系密切。通过分析研究区Te与地热场(地表热流、居里面深度)、地震之间的关系,本文得到如下认识:(1)Te与地热场参数具有较好的相关性,但受浅部地壳被破坏,深部仍为克拉通地壳影响,导致龙门山断裂带和江南造山带区域的Te与地表热流或居里面深度之间的部分对应关系相反。(2)Te与地震关系复杂,Te较薄区域并不代表着地震频发区域,地震活动性与其所处的深部环境相关。龙门山断裂带强震频发的原因是受周边两块体中上地壳刚性地层长期相互作用,致使应力和能量积累较强;华夏地块区域地震较少是因为深部热物质上涌对华夏地块的壳幔进行强烈...     

内蒙古狼山地区新元古代狼山群变形特征及区域构造意义

狼山群作为阿拉善地块东北部重要的前寒武系地层单元,记录了复杂的构造变形演化历史。本文通过对狼山西南段出露的新元古代狼山群进行构造地质试点填图、剖面测制等方面的研究,总结了狼山群构造变形特征,并对其变形时代和期次进行了初步划分。研究表明狼山群第一期构造变形特征表现为一系列同斜倒转褶皱构造(D1),第二期为近东西向的韧性右行走滑剪切(D2),第三期为北东向巴彦乌拉山—狼山断裂活动对狼山群构造变形进行改造(D3)。结合该地区前人研究成果及年代学资料认为:新元古代狼山群同斜倒转褶皱主要受到古亚洲洋向南俯冲,在古生代晚期沿恩格尔乌缝合带与阿拉善地块北缘拼合,导致近南北向的构造挤压,在狼山地区形成了早期枢纽NEE-EW向的褶皱;晚古生代末期-中生代早期古亚洲洋关闭后发生的板内变形在狼山地区形成韧性右行剪切;三叠纪中晚期韧性左行剪切形成北东向走滑断裂,并使早期形成的褶皱枢纽走向转为北东,奠定了狼山地区现今的构造格局。     

柯坪块体周缘主要断裂闭锁程度与滑动亏损速率的分布特征

利用西南天山山前柯坪块体周缘的多期中国大陆构造环境监测网络区域站及新疆维吾尔自治区地震局在该区域加密布设的多个GPS站点数据,基于负位错反演原理,采用Defnode软件反演,得到柯坪、皮羌和迈丹3条主要断裂的闭锁程度和滑动亏损速率分布特征.模拟结果与实测值总体一致,但略有细微差异,GPS点位密集处残差在1mm/a左右,...     

云开地区变质基底锆石U-Pb年代学及对华夏地块Grenvillian事件的指示

云开地区前寒武纪变质基底主要是以片麻岩类为代表的高州杂岩和以浅变沉积岩类为代表的云开群,均不同程度受到了后期加里东期和印支期构造-岩浆事件的改造。对高州杂岩中1个变粒岩(YK1016)和1个黑云斜长片麻岩(YK1014-3)、云开群中1个黑云斜长片麻岩(YK1017-1)样品进行了锆石LA-ICP-MS U-Pb定年。样品YK1016中获得的不一致曲线上下交点年龄分别为(1 035±43)Ma和(437±30)Ma(n=16,MSWD=1.3),上交点年龄与10个谐和点207Pb/206Pb加权平均年龄(970±30)Ma(MSWD=0.22)一致,解释为其形成年龄。样品YK1014-3获得的206Pb/238 U表观年龄范围为949~2 395 Ma,主要集中在1 000~1 200 Ma,9个最年轻且谐和的分析点207Pb/206Pb加权平均年龄为(970±32)Ma(MSWD=0.53),对其最大沉积年龄进行了约束。样品YK1017-1获得的206Pb/238 U表观年龄范围为650~2 383 Ma,也主要集中在1 000~1 200 Ma,最年轻且谐和的碎屑锆石年龄将其时代约束在650 Ma。结合云开地区变质基底中已有的高精度年龄数据,认为高州杂岩形成时代为中元古代晚期—新元古代早期,云开群的形成时代为新元古代晚期。另外,研究结果及云开地区已有的数据表明,该区除出露Grenville期岩石外,新元古代变沉积岩中还存在大量Grenville期(900~1 350 Ma)碎屑锆石,部分晶形较好具岩浆起源,这一现象从云开地区一直延伸到华夏东部;但在华夏地块由南往北,Grenville期锆石呈减少趋势,新元古代锆石(820~860 Ma)逐渐增多。这些暗示华夏地块南缘存在一个由南东向北西推进的Grenvillian造山带,和(或)在华夏地块内部存在略晚于Grenville期的构造-岩浆事件。