Early Callovian ingression in southwestern Gondwana. Palaeoenvironmental evolution of the carbonate ramp (Calabozo Formation) in southwestern Mendoza,Neuquen basin,Argentina

The carbonatic sequence of the Calabozo Formation (Lower Callovian) developed in southwestern Gondwana, within the northern area of the Neuquén basin, and is widespread in thin isolated outcrops in southwestern Mendoza province, Argentina. This paper describes the facies, microfacies and geochemical-isotopic analysis carried out in five studied localities, which allowed to define the paleoenvironmental conditions of a homoclinal shallow ramp model, highly influenced by sea level fluctuations, where outer, mid and inner ramp subenvironments were identified. The outer ramp subenvironment was only recognized in the south of the depocenter and is characterized by proximal outer ramp facies with shale levels and interbedded mudstone and packstone layers. The mid ramp subenvironment is formed by low energy facies (wackestone) affected by storms (packstones, grainstones and floatstones). The inner ramp subenvironment is the most predominant and is characterized by tidal flat facies (wackestones, packstones and grainstones) over which a complex of shoals (grainstones and packstones) dissected by tidal channels (packstone, grainstones and floatstones) developed. In the north area, protected environment facies were recorded (bioturbated wackestones and packstones). The vertical distribution of facies indicates that the paleoenvironmental evolution of the Calabozo Formation results from a highstand stage in the depocenter, culminating in a supratidal environment, with stromatolitic levels interbedded with anhydrite originated under restricted water circulation conditions due to a progressive isolation of the basin. δ¹³C and δ¹⁸O values of the carbonates of the Calabozo Formation suggest an isotopic signature influenced by local palaeoenvironmental parameters and diagenetic overprints. The δ¹³C and δ¹⁸O oscillations between the carbonates of the different studied sections are related with lateral facies variations within the carbonate ramp accompanied with dissimilar reactivities in relation to diagenetic fluids. The δ¹⁸O values of all sections exhibit a rather broad scatter which may be attributed to diagenesis and recrystallisation while the carbon isotopic composition has been less affected by those processes. Carbon isotope system has best retained the primary isotopic signal and δ¹³C values (0–3.9‰) are within the Callovian isotope range. The ⁸⁷Sr/⁸⁶Sr ratios of the bulk carbonates of El Plomo creek, La Vaina creek and Potimalal River sections are in agreement with the Callovian seawater Sr-isotope curve.     

Neotectonics of East Anatolian Plateau (Turkey) and Lesser Caucasus: implication for transition from thrusting to strike-slip faulting

Abstract

The east Anatolian plateau and the Lesser Caucasus are characterised and shaped by three major structures: (1) NW- and NE-trending dextral to sinistral active strike-slip faults, (2) N-S to NNW-trending fissures and /or Plio-Quatemary volcanoes, and (3) a 5-km thick, undeformed Plio-Quatemary continental volcanosedimentary sequence accumulated in various strike-slip basins. In contrast to the situation in the east Anatolian plateau and the Lesser Caucasus, the Transcaucasus and the Great Caucasus are characterised by WNW-trending active thrust to reverse faults, folds, and 6-km thick, undeformed (except for the fault-bounded basin margins) continuous Oligocene-Quaternary molassic sequence accumulated in actively developing ramp basins. Hence, the neotectonic regime in the Great Caucasus and the Transcaucasus is compressional-contractional, and Oligocene-Quaternary in age; whereas it is compressional-extensional, and Plio-Quatemary in age in the east Anatolian plateau and the Lesser Caucasus.

Middle and Upper Miocene volcano-sedimentary sequences are folded and thrust-to-reverse-faulted as a result of compressional- contractional tectonic regime accompanied by mostly calc-alkaline volcanic activity, whereas Middle Pliocene-Quaternary sequences, which rest with angular unconformity on the pre-Middle Pliocene rocks, are nearly flat-lying and dominated by strike-slip faulting accompanied by mostly alkali volcanic activity implying an inversion in tectonic regime. The strike-slip faults cut and displace dykes, reverse to thrust faults and fold axes of Late Miocene age up to maximum 7 km: hence these faults are younger than Late Miocene, i.e., these formed after Late Miocene. Therefore, the time period between late Serravalian (~ 12 Ma) continent-continent collision of Arabian and Eurasian plates and the late Early Pliocene inversion in both the tectonic regime, basin type and deformation pattern (from folding and thrusting to strike-slip faulting) is here termed as the Transitional period.

Orientation patterns of various neotectonic structures and focal mechanism solutions of recent earthquakes that occurred in the east Anatolian plateau and the Caucasus fit well with the N-S directed intracontinental convergence between the Arabian plate in the south and the Eurasian plate in the north lasting since Late Miocene or Early Pliocene in places. © 2001 Éditions scientifiques et médicales Elsevier SAS     

The role of thrust ramp reactivation in pull-apart mechanism of the Erzincan basin,North Anatolian Fault Zone,Turkey

The new field data obtained from the southwestern margin of the Erzincan pull-apart basin located on the eastern segment of North Anatolian Fault Zone indicate that the opening of the basin is not only controlled by pull-apart mechanism but also by a lateral ramp structure associated with SSE-NNW Late Miocene thrusting along the Sivas Basin. The fault bordering the southwestern margin of the basin is the lateral part of the Karada thrust that is the roof thrust of the Sivas fold-thrust system, rather than a segment of the North Anatolian Fault Zone. The Erzincan basin was nucleated as a lateral ramp basin during the Pliocene on the lateral ramp-related folds and expanded by the pull-apart opening mechanism between two segments of the North Anatolian Fault Zone. The WSW-ENE pull-apart opening of the basin was recorded by the Pliocene lacustrine-fluvial sediments and Quaternary volcanics as listric normal faulting.     

广东凡口MVT铅锌矿床成矿年代——来自辉绿岩锆石SHRIMP定年证据

广东凡口铅锌矿床是我国超大型铅锌矿床之一,已探明铅锌金属含量已达千万吨.矿床赋存于华南泥盆系海进序列中上部的白云质碳酸盐岩中,矿体呈不规则状受断裂构造控制,具有密西西比河谷型(MVT)矿床特点.矿区发育辉绿岩脉,多呈NWW向,部分呈NNW向,靠近矿体的辉绿岩全部粘土化和碳酸盐化.辉绿岩穿切硫铁铅锌矿,同时辉绿岩中也含有浸染状铅锌矿细脉,显示辉绿岩形成于成矿作用的晚期,成矿作用持续时间长,辉绿岩就位时间短,就位后成矿作用仍在继续,因此,辉绿岩就位年龄可基本代表成矿作用年代.辉绿岩中锆石SHRIMP年龄主要集中于2213～957Ma、341～216Ma、138～127Ma、122～90Ma 4个区间,前二组代表捕获围岩的锆石.通过凡口矿床地质特点、区域地质演化以及与区域内铀矿、辉绿岩年龄数据的比较,122～90Ma组代表了凡口辉绿岩的侵位时代,相当于早白垩世晚期,显示铅锌矿化形成于华南板内拉张构造环境,与该时期发育大规模红层盆地相对应.由此建立区域(凡口式)铅锌矿化模型:白垩纪始,随着五夷山的隆起,阻断了太平洋暖湿气流的西进,华南地区开始了红层盆地发育阶段,铅锌矿主要分布于这些红层盆地周边地区;来自红层盆地的盆地卤水与来自生烃层的富硫的还原性卤水混合,形成铅锌矿床.     

昌都地块小苏莽一带二叠纪变玄武岩地球化学特征及其构造环境

宁多岩群作为北羌塘昌都地块的变质岩系之一,其形成时代的有效限定便显得至关重要.通过高精度的LA-ICP-MS(激光剥蚀等离子体质谱)锆石微区原位U-Pb同位素测年,在青海省玉树县小苏莽乡宁多村北原划为中新元古代宁多岩群的变玄武岩中获得原岩形成年龄为(272.88±0.48) Ma,将该套变玄武岩从宁多岩群中解体出来,划归中二叠统.变玄武岩具有高Ti、富Fe、偏碱的特征,呈现无Eu异常、轻稀土富集的右倾型稀土元素分布模式,微量元素分布模式为部分大离子亲石元素和高场强元素不同程度富集的大隆起型曲线.综合判断,该变玄武岩形成于大陆裂谷环境,可能与地幔柱活动有关,是羌塘地区二叠纪大规模陆内裂谷作用的地质记录.     

解析法图像重建中的理想斜变滤波器的进一步研究(英文)

理想斜变滤波器是一种被傅里叶逆变换定义的广义函数。设计理想斜变滤波器的实用数字滤波器是最重要的任务之一。推导了理想斜变滤波器的单位冲激响应的公式。提出了两种实用数字滤波器的设计模式:连续模式和离散模式。分析了3种已经存在的实用滤波器,设计了2种新的实用滤波器。通过一个仿真实验,比较了这5种滤波器。总的来说,以离散模式设计的离散方法优于以连续模式设计的连续方法。对于2种离散方法来说,无限长离散方法优于自适应长度离散方法,它们均可以得到较好的重建质量。对于3种连续方法而言,线性方法是最好的,可以重建出工程可用的CT图像,而常数方法和抛物线方法不能重建出可用的CT图像。     

大渡河上游丹巴地区地貌分维特征的地质解析

基于高精度DEM,采用滑动窗口技术,计算丹巴地区的地貌分形特征.结果表明:地质构造决定地貌的宏观格架,构造规模和期次与地貌的平均分维值和标准差呈正相关关系;地层岩性决定地貌的中观和微观布局,变质作用与平均分维值和最大分维值呈正相关关系;地貌维度的极大值区多为地质环境异常区.该项研究对山地本体认知、地质环境调查及评价具有重要的参考价值.     

Geomorphological palaeoenvironments of the Sneeuberg Range,Great Karoo,South Africa

Sedimentary sequences within headwater valleys on the landward side of the Great Escarpment of South Africa are elucidated and their significance as indicators of environmental change is assessed. This study focuses on the Sneeuberg Range, the most prominent mountain range in the semi‐arid central Great Karoo. Valley fills of a hitherto unrecognised complexity and of a greater age than any previously recorded in the central Great Karoo are reported. Three phases of deposition spanning the Late Pleistocene up to the present are documented from sites where gully erosion has incised the valley fills. The earliest depositional phase is represented by deeply weathered, calcretised gravel deposits, which probably were emplaced by debris flow and fluvial processes in the form of a fan. These deposits subsequently were buried by finer grained, largely unconsolidated sediment, with much of this emplacement occurring during the Holocene. There is evidence for phases of landscape stability and instability within this facies. Finally, sheetwash has removed fine‐grained sediments from valley flanks and has deposited it either on valley bottoms, or in presently active gullies. This process appears to be ongoing, and is the subject of current investigation. The sedimentary deposits are interpreted as representing a wide range of palaeoenvironmental conditions that have prevailed within the central Great Karoo since the penultimate glaciation. Copyright © 2003 John Wiley & Sons, Ltd.     

Late Quaternary depositional history of the Reuss delta,Switzerland: constraints from high‐resolution seismic reflection and georadar surveys

Glacial erosion has caused overdeepening of many alpine valleys. After retreat of the ice, they were filled with heterogeneous deposits of glacial, lacustrine and fluvial sediments. A typical example of such a valley segment and its infill is the Reuss delta on the southern shore of Lake Lucerne in Switzerland. To obtain a detailed three‐dimensional image of this valley segment, the ETH Institute of Geophysics has acquired several two‐dimensional, high‐resolution seismic and georadar profiles, and conducted a three‐dimensional georadar survey. Interpretations of these geophysical data were constrained by a geological core extracted from a borehole 300 m deep near the investigation site. The seismic profiles imaged ca. 600 m of sediment infill above bedrock. Based on their reflection characteristics, five different deposition units were distinguished. These units were interpreted as a succession of clay/silt at the base, followed by different sand units with variable but generally increasing amounts of gravel. This succession represented a prograding delta that filled the southern part of Lake Lucerne. The latest fluvial development of the region is best represented by the georadar data. In particular, the three‐dimensional georadar data set provides a detailed view of an ancient braided river channel Copyright © 2002 John Wiley & Sons, Ltd.     

青藏高原东缘1933年叠溪Ms7.5级地震发震构造再研究

青藏高原东缘1933年叠溪75级地震的发震构造至今仍然难以琢磨,前人或将其归因于NW向松坪沟断裂的左旋走滑活动、或南北向岷江断裂左旋走滑活动,但地表同震破裂、地震地质、地震等烈度图等调查和研究结果都不支持这种走滑型断层的地震成因。本文基于叠溪地震区构造地貌和湖相地层断层调查,结合古地震和历史地震的研究结果,提出了与2013年四川芦山Ms 70级地震类似的发震构造模型,即隐伏断坡型逆冲断层发震构造模型,认为在叠溪震区10~15km深部隐伏一条西倾的逆冲断坡,其向东逆冲作用导致了叠溪地区频繁的地震活动。这个发震模型有待深部地球物理测深资料和地表大地测量资料的验证。