Paleomagnetic and gravimetrical reconnaissance of Cretaceous volcanic rocks from the Western Colombian Andes: paleogeographic connections with the Caribbean Plate

The reconstruction of the tectonic evolution of the oceanic crust, including the recognition of ancient oceanic plumes and the differentiation between multiple and single oceanic arcs, relies on the paleogeographic analysis of accreted oceanic fragments found in orogenic belts. Here we present paleomagnetic and gravity data from Cretaceous oceanic basaltic and gabbroic rocks, the continental metamorphic basement, and their associated cover from northwestern Colombia. Based on regional scale tectonic reconstructions and geochemical constraints, such rocks have been interpreted as remnants of an oceanic large igneous province formed in southern latitudes, which was accreted to the sialic continental margin during the Late Cretaceous. Gravity analyses suggest the existence of a coherent high density segment separated by major suture zones from a lower density material related to the continental crust and/or thick sedimentary sequences trapped during collision. A characteristic paleomagnetic direction in Early and Late Cretaceous oceanic volcano-plutonic rocks, revealing a southeastern declination (D) and a negative inclination (I), may be interpreted in two different ways: (1a primary magnetization (tilt-corrected direction D = 130.3°, I = -23.3°, k = 23.4, α₉₅ = 26.4°), suggesting clockwise rotation around 130°, and magnetization acquired in southern latitudes (range of 4°S to 21°S); or (2) a remagnetization event during a reverse interval of the Earth’s magnetic field in the Cenozoic (in situ direction D = 128.7°, I = -6.2°, k = 23.1, α₉₅ = 26.1°), suggesting a counter-clockwise rotation around 50°. The first scenario seems more plausible, as it is consistent with previous paleomagnetic studies at other localities; it is compatible with a southern paleogeography for this block, and when integrated with other regional geological and paleomagnetic studies, supports a southern Pacific origin of a major oceanic block, formed as a part of a broader Cretaceous plateau that may have extended south or southwest of Galapagos. After its initial accretion, this block was subsequently fragmented due to the oblique SW-NE approach to the continental margin during the Late Cretaceous.     

江西崇义淘锡坑大型钨矿床成矿花岗岩体研究

简介了江西崇义淘锡坑大型钨矿床的地质背景、矿化特征,详细叙述了淘锡坑岩体的形态、分布、矿物成分、化学成分,归纳了最新的同位素年龄及地球化学数据,探讨了淘锡坑岩体作为成矿母岩的成矿作用,并对其整体形态进行了预测,分析了下步的找矿有利地段。     

Parasequence development in the Ediacaran Shuram Formation (Nafun Group, Oman): high-resolution stratigraphic test for primary origin of negative carbon isotopic ratios

Neoproterozoic carbonates are known to show exceptional variations in their carbon isotopic ratios, and in the absence of biostratigraphy and a firm geochronological framework, these variations are used as a correlation tool. However, it is controversial whether the carbon isotope record reveals a primary oceanographic signal or secondary effects such as diagenesis. The Shuram Formation of the Nafun Group of Oman allows a stratigraphic test of this problem. The Nafun Group (Huqf Supergroup, Oman) in the Huqf area of east-central Oman consists of inner carbonate ramp facies of the Khufai Formation overlain by marine, storm-generated, red and brown siltstones of the Shuram Formation. Towards its top, the Shuram Formation is composed of distinctive shallowing-upward, 4–17-m-thick parasequences cropping out continuously over 35 km, which show recessive swaley cross-stratified siltstones capped by ledges comprising wave-rippled, intraclast-rich ooidal carbonate. These storm-dominated facies show a regional deepening in palaeobathymetry towards the south. The carbonates of the Shuram Formation are marked by an extreme depletion in ¹³C in bulk rock. δ¹³C values quickly reach a nadir of −12‰ just above the Khufai-Shuram boundary and steadily return to positive values in the overlying mainly dolomitic Buah Formation. The Shuram excursion is thought to be ca . 50 Myr in duration and extends over 600 m of stratigraphy. Carbon isotopic values show a systematic variation in the parasequence stack, with values varying both vertically through the stratigraphy (∼2‰ per 45 m) and laterally in the progradation distance (∼1‰ over 35 km). This supports a primary, oceanographic origin for these extremely negative carbon isotopic values and independently argues strongly against diagenetic resetting.     

Assessment of observational networks with the Representer Matrix Spectra method—application to a 3D coastal model of the Bay of Biscay

The development of coastal ocean modeling in the recent years has allowed an improved representation of the associated complex physics. Such models have become more realistic, to the point that they can now be used to design observation networks in coastal areas, with the idea that a “good” network is a network that controls model state error. To test this ability without performing data assimilation, we set up a technique called Representer Matrix Spectra (RMS) technique that combines the model state and observation error covariance matrices into a single scaled representer matrix. Examination of the spectrum and the eigenvectors of that matrix informs us on which model state error modes a network can detect and constrain amidst the observation error background. We applied our technique to a 3D coastal model in the Bay of Biscay, with a focus on mesoscale activity, and tested the performance of various altimetry networks and an in situ array deployment strategy. It appears that a single nadir altimeter is not efficient enough at capturing coastal mesoscale physics, while a wide swath altimeter would do a much better job. Testing various local in situ array configurations confirms that adding a current meter to a vertical temperature measurement array improves the detection of secondary variability modes, while shifting the array higher on the shelf break would obviously enhance the model constraint along the coast. The RMS technique is easily set up and used as a “black box,” but the utility of its results is maximized by previous knowledge of model state error physics. The technique provides both quantitative (eigenvalues) and qualitative (eigenvectors) tools to study and compare various network options. The qualitative approach is essential to discard possibly inconsistent modes.     

基于“嫦娥一号”干涉成像光谱数据的月表虹湾地区FeO及TiO_2含量评估

基于"嫦娥一号"干涉成像光谱数据对月表虹湾地区元素分布进行分析。首先对该数据进行辐射畸变校正、反射率反演和几何校正等预处理;然后基于Apollo月球样品含量的实验室实测数据,建立一套基于"嫦娥一号"干涉成像光谱数据的FeO及TiO2含量反演模型;最后计算获取月表虹湾地区FeO及TiO2含量的分布图,并针对分布结果对虹湾地区的成因提出假设。     

A Method for Calculating Bottom Backscattering Strength Using Omnidirectional Projector and Omnidirectional Hydrophone

A method of obtaining bottom backscattering strength by employing an omnidirectional projector and omnidirectional hydrophone is proposed. The backscattering strength is extracted from monostatic backscattering data. The method was adopted in an experiment conducted in the South Yellow Sea of China. The seafloor surface was relatively smooth and covered by a small quantity of shell fragments, as observed through a digital camera system. Sampling data showed that the main component of the sediment at this experimental site was fine sand. In this paper, we detail the calculation method. Preliminary results of backscattering strength as a function of grazing angle(20?–70?) in the frequency range of 6–24 kHz are presented. The measured backscattering strength increased with the grazing angle and changed more rapidly at large grazing angles(60?–70?). A comparison of the data at different frequencies reveals that the measured backscattering strength substantially rises with the increase of acoustic frequency. A fitting curve of Lambert's law against the measured data shows that the backscattering strength deviates from Lambert's law at large grazing angles.     

Cryologic continuum of a steep watershed

Ice processes taking place in steep channels are sensitive to the thermal and hydrological regimes of upstream reaches and tributaries as well as to the local channel morphology. This work presents freezeup, mid‐winter, and breakup data from four channels of increasing order located in a cold temperate watershed during the winter 2010–2011. From headwater channels to the main drainage system, water temperature, ice coverage, and ice processes are reported and related to weather conditions and to channel characteristics. Headwater channels only formed ephemeral ice features, and their water temperature reached as much as 4 °C in mid‐winter. On the other hand, larger channels formed impressively large ice dams, some of them reaching 2 m in height. The development of a suspended ice cover partially insulated the channels; as a result, water temperatures remained above 0 °C even for air temperatures well below freezing. This work presents steep channels ice processes that have not been described in previous publications. The concept of a watershed cryologic continuum (WCC) is developed from the data collected at each channel order. This concept emphasizes the feedback loops that exist between morphology, hydrology, heat, and ice processes in a given watershed and can lead to a better understanding of ice processes taking place at any channel location within that watershed. The WCC can also contribute in improving our understanding of the impacts of climate change on the cryologic and thermal regimes of steep channels. Copyright © 2012 John Wiley & Sons, Ltd.     

使用SAR影像生成DEM的精度分析

雷达干涉测量(InSAR)和雷达立体测量(Radargrammetry)是使用SAR影像生成DEM的两种主动遥感新技术,前者利用反射雷达波的相位信息,而后者利用其灰度信息。本文利用ERS-1/2卫星对香港地区成像获取的SAR影像分别构成干涉像对和立体像对生成了DEM,并以外部DEM为参考,利用对比和统计方法分析了两种测量方法的精度情况和技术优势。     

南雄盆地始新统红层沉积环境分析

选取南雄盆地始新统红层——古城村剖面作为研究对象,结合磁性矿物和地球化学元素特征分析其沉积环境。古城村剖面上部和下部为红色,中部为青灰色,颜色及其深浅均随沉积层理/层次逐渐变化,说明颜色和沉积地层层理/层次为同期形成,为地层的原生色。磁学测量的饱和等温剩磁（SIRM）和剩磁矫顽力（Bcr）结果与红度（a*）呈现良好的正对应关系,热磁曲线结果表明红层样品以赤铁矿为主,漫反射光谱（DRS）佐证了原生赤铁矿的存在且是唯一可以检测到的磁性矿物。赤铁矿指示了当时的沉积环境为高温少雨。剖面中部存在青灰色地层,该层样品的热磁测量没有检测到磁性矿物信号,只检测到顺磁性矿物信号。高温处理后的DRS测量也表明其存在顺磁性的黏土矿物。通过地球化学元素分析显示其与标准石灰岩相比差异较大,而与中国典型黄土较为相似,可能说明青灰层并非石灰岩。剖面的上下层均为以原生赤铁矿为主的红层,代表高温强氧化环境。青灰层与上下红层之间为过渡渐变,地层较为连续且无不整合现象,化学元素分析也显示该层与上下红层、上地壳和黄土都具有较高的相似性,所以该层更可能是风积物在气候转为湿润条件下经成土作用形成的钙质淀积层。古城剖面反映了始新世时期干热氧化—湿润氧化还原—干热氧化的逐渐变化过程,对于准确理解南雄盆地此阶段古环境有重要意义。