Simultaneous horst-basin formation and magmatism during Late Variscan transtension: evidence from 40Ar/39Ar and 207Pb/206Pb geochronology in the Ruhla Crystalline Complex

40 Ar/³⁹Ar–mica and ²⁰⁷Pb/²⁰⁶Pb–zircon dates are presented and combined with existing P–T data and the sedimentary record. These data indicate that the RCC was faulted into three segments which underwent different exhumation histories during the Late Carboniferous/Early Permian. The eastern segment shows ⁴⁰Ar/³⁹Ar–biotite data of336 ±4 and 323±3 Ma. Furthermore, it is intruded by the Thuringian Hauptgranite dated at 337±4 Ma by the ²⁰⁷Pb/²⁰⁶Pb single zircon method. At approximately 300 Ma rocks of the eastern segment were finally exposed and, subsequently, subsided as part of the Oberhof pull-apart basin, filled by Late Carboniferous/Early Permian molasse sediments and volcanic rocks (296–285 Ma; Goll 1996). A similar Late Carboniferous evolution is inferred for the western segment, since it is also overlain by Upper Carboniferous volcanic rocks. In contrast to the eastern and western segments, distinctly younger intrusion and cooling ages were recorded for the central segment of the RCC (⁴⁰Ar/³⁹Ar muscovite: 311±3 Ma; ⁴⁰Ar/³⁹Ar biotite: 293–288±3 Ma) that was intruded by the Trusetal Granite, the Ruhla Granite and Brotterode Diorite (²⁰⁷Pb/²⁰⁶Pb single zircon: 298±2, 295±3, 289±4 Ma, respectively). These young data are unique in the MGCR and testify that plutonic activity and cooling of basement rocks took place simultaneously with basin formation and volcanism in the eastern and western segments. Overlying Upper Permian (Zechstein) and Triassic sediments indicate final exposure of the central segment by approximately 260 Ma, as a part of the Ruhla-Schleusingen Horst. Combination of these results with P–T data from the contact aureole of the Trusetal granite indicate that the central segment was unroofed by at least 8.5 km during the Late Carboniferous. The Late Carboniferous/Early Permian horst-basin formation, documented in the RCC, is due to dextral transtensional movements along the NW-trending Franconian fault system. It may have been enhanced by mantle upwelling widespread in Central Europe during the Early Permian that also caused intensive magmatism in the Thuringian Forest region. Received: 2 February 1999 / Accepted: 15 November 1999     

Coastal Changes Over the Past 200 Years Around Ovalau and Moturiki Islands,Fiji: Implications for coastal zone management

The islands Ovalau and Moturiki in central Fiji are selected for the investigation of coastal change over the past c. 200 years. Although having coastal environments typical of many tropical Pacific Islands, Ovalau and Moturiki are also atypical because they experienced urban and infrastructural developments before most other parts. The associated records enable recent coastal changes to be discerned more clearly here than has been possible elsewhere. The islands are surrounded by coral reef, the configuration of which accounts for variations in vulnerability of their coasts to erosion. Interviews were conducted in each of 22 settlements along the islands' coasts and information obtained about recent coastal change. Mangroves are concentrated along leeward coasts, although they have been cleared from many windward coasts in the last 40 + years, causing shoreline erosion to be initiated/accelerated. Most shoreline-protection initiatives (vegetation planting and seawall construction) have failed. Three major management implications of the study are discussed. Firstly, there is a need to redefine the nature of the interactions between coastal inhabitants and coastal ecosystems, so that environments are sustainably developed; and specifically that reefs are conserved and shoreline vegetation (especially mangroves) is effectively replanted. Secondly, information about appropriate design and composition of artificial structures for shoreline protection needs to be made available to the local communities who construct most of them. Thirdly, alternative sources of sand and rock aggregate to those whose extraction aggravates shoreline erosion should be sought.     

Age and life histories of black and smooth oreos inferred using bomb radiocarbon dating and stable isotope analysis

Black oreo (Allocyttus niger) and smooth oreo (Pseudocyttus maculatus) are commercially exploited deepwater fishes in New Zealand. They are considered to be long-lived with unvalidated maximum otolith growth zone counts of 153 years and 86 years, respectively. Bomb radiocarbon dating results supported the otolith age estimates for A. niger, but those for P. maculatus provided only partial support. Oxygen isotope (δ¹⁸O) results for A. niger otoliths suggested that juveniles were in near-surface waters and adults in depths of 700–1300 m. In contrast, P. maculatus otoliths suggested a variable depth history (500 to at least 1500 m) throughout life. Carbon isotope (δ¹³C) results showed a steady enrichment from core values to c. 20 years for both species, owing to changes in diet and metabolic rate from the juvenile to the adult fish, associated with a depth increase. Von Bertalanffy growth parameters are provided for both species.     

Temperature fluctuations and current shear in Antarctic Bottom Water at the Vema Sill

The Vema Channel acts as a major conduit for the equatorward spreading of Antarctic Bottom Water between the Argentine and Brazil Basins. For almost two years the thermal stratification above its saddle depth (4660 m) – called Vema Sill – was recorded by moored thermistors and current meters. The lowest 490 m of the water column was instrumented to monitor the well-developed benthic boundary layer of Antarctic Bottom Water. The latter can be subdivided into Weddell Sea Deep Water on the sea bed and lower Circumpolar Deep Water above it. The data show fluctuations on various scales including periods, each about 1–2 weeks long, when the abyssal stratification virtually disappeared. Assuming a stable ratio between density and temperature, time series of bulk Richardson numbers are estimated from temperature and current shear data. The results suggest a potential for intermittent episodes of locally generated vertical mixing.     

中承式钢筋混凝土拱桥自振特性分析

利用有限元通用软件ANSYS,对一中承式钢筋混凝土拱桥进行了结构自振特性的数值模拟分析,求出了自振频率、振型等动力参数,对其模态特征进行了描述,并通过变换拱桥的主要结构参数对比分析了它们对结构自振特性的影响.结果表明:自振频率随矢跨比的减小略有提高;横撑有助于增强拱肋的横向刚度,减小拱肋面外振动,提高抗风稳定性;各振型的自振频率值随拱肋刚度的提高近似成线性增大,可以通过改变拱肋刚度来调整其自振特性.研究结果可以为同类桥梁的抗震设计提供参考.     

东印度洋Roo海隆区域属性及其俯冲区域响应特征

东南亚巽他弧形汇聚板块俯冲构造体系是全球最为活跃的板块汇聚边界地带之一, 早期研究认为巽他弧俯冲体系主要发育俯冲增生型板块边缘, 以典型的增生棱柱体弧前隆起地貌为主要特征; 最新研究发现东印度洋沃顿海盆圣诞岛海底火山群省最东段Roo海隆已经随板块运动到达爪哇海沟区域, 与巽他弧其他区域正常洋壳俯冲过程相比, Roo海隆凸起构造在巽他弧体系中触发了不同的俯冲地质过程.本文结合前人研究成果, 全面梳理认识Roo海隆区域属性特征, 包括海隆岩性与年代学特征、起源争议、"隆-沟"区域俯冲深部结构特征; 进一步增强对弧前早期俯冲侵蚀过程的理解, 包括局部增生棱柱体前缘碰撞侵蚀凹陷、弧前隆起差异性抬升、弧前盆地挤压破碎变窄; 并首次利用二维多道地震资料解释, 探讨弧后盆地对"隆-沟-弧-盆"新俯冲构造格局的响应特征.目前弧后肯登-马都拉海峡盆地内正在发生新一期挤压构造运动, 我们认为其发育的浅层挤压背斜构造特征是Roo海隆凸起构造形成的垂直正交快速高角度新俯冲构造格局下弧后盆地内的直接构造变形响应.     

南大洋深水沉积物波举例研究

深水细粒底流型沉积物波多分布于陆坡、洋盆等深水洋底。以大澳大利亚湾上部陆坡和康拉德隆起西南坡的波状底形为例,解释了南大洋深水沉积物波的形态特征、分布特点和形成发育机理。以大澳大利亚湾的沉积物波为代表的钙质细粒底流型沉积物波波高约40 m,波长1~2 km,直线型脊线长10 km,平行于等深线,主要由苔藓虫软泥组成,间冰期陆架上苔藓虫丰富,冰期移至陆架外缘和陆坡上部,在流速约16 cm/s的高密度出流作用下于陆坡上部发育成泥质沉积物波。康拉德隆起西南坡的硅质细粒底流型沉积物波由硅藻软泥组成,绕南极流锋面的上升流使得此处的生产力高,硅藻类微体生物丰富,在流速约6 cm/s的底流作用下发育大量沉积物波。2种细粒底流型沉积物波的向陆侧翼的沉积率大于向海侧翼,披覆形成后爬不同相爬升层理,显示沉积物波向上游坡迁移。     

鲁西地区奥陶系碳酸盐岩储集特征及主控因素

应用薄片、铸体和阴极发光等室内分析技术,对鲁西隆起奥陶系碳酸盐岩及济阳坳陷潜山奥陶系钻井岩心样品进行了成岩作用及储集特征详细研究。奥陶系碳酸盐岩原生孔隙消失殆尽,次生成因的孔、洞和缝是主要储集空间。其中裂缝储集层在各层段均有发育,但白云岩段优于其它岩性段;构造缝主要分布在构造轴部和断裂带附近,溶蚀缝在不整合面附近较为发育。孔洞型储集层主要发育在白云岩层段,以晶间孔、晶间溶孔和晶内溶孔为主要储集空间;有少量有效孔隙分布在灰岩段,以铸模孔、粒内溶孔和粒内孔为主。马家沟组八陡段顶部发育溶蚀角砾岩、纹层状结晶灰岩,砾间缝、晶间缝和溶洞为主要储集空间。白云岩段是奥陶系碳酸盐岩主要储集层段,构造因素、岩性和成岩作用(尤其是胶结作用和溶蚀作用)是造成奥陶系碳酸盐岩储集层剖面分布差异性的主要控制因素。