Analyse statistique des séquences sèches dans un bassin du nord de la Tunisie / Statistical analysis of dry events in a northern Tunisian basin

Abstract

A comparative statistical study of dry events during the rainy season is presented. In particular, we carried out statistical processing of the daily records of raingauges in the downstream basin of Lake Ichkeul, in the north of Tunisia. The climate is characterized by a rainy season lasting from the autumn until spring. The Poisson distribution was applied to describe the number of rainfall events, and negative binomial distribution was applied for the length of the dry events, in the rainy season. Since rainfall events are shorter, their duration follows a geometrical distribution, as theoretically required. For planning purposes, the longest seasonal dry spells associated with the various statistical recurrence periods are derived on the basis of the fitted GEV functions. A hydrological year starts at the beginning of the first rainfall event of a given season. The length of hydrological year is determined by the time interval between the start dates of two subsequent rainy seasons. The beginning of the hydrological year occurs on average toward mid-September, but the probability of it occurring before 15 September exceeds 40%. Spatial analysis of dry events is also done. A significant fraction of the dry events occurred for at least two stations simultaneously. Furthermore, 4.5% of the dry events have been observed at all three stations. The analysis of the dry events gives an alternative method to examine the dry spell phenomenon.     

Geosocial gauge: a system prototype for knowledge discovery from social media

The remarkable success of online social media sites marks a shift in the way people connect and share information. Much of this information now contains some form of geographical content because of the proliferation of location-aware devices, thus fostering the emergence of geosocial media – a new type of user-generated geospatial information. Through geosocial media we are able, for the first time, to observe human activities in scales and resolutions that were so far unavailable. Furthermore, the wide spectrum of social media data and service types provides a multitude of perspectives on real-world activities and happenings, thus opening new frontiers in geosocial knowledge discovery. However, gleaning knowledge from geosocial media is a challenging task, as they tend to be unstructured and thematically diverse. To address these challenges, this article presents a system prototype for harvesting, processing, modeling, and integrating heterogeneous social media feeds towards the generation of geosocial knowledge. Our article addresses primarily two key components of this system prototype: a novel data model for heterogeneous social media feeds and a corresponding general system architecture. We present these key components and demonstrate their implementation in our system prototype, GeoSocial Gauge.     

Permian Peri‐glacial Deposits from Central Mongolia in Central Asian Orogenic Belt: A Possible Indicator of the Capitanian Cooling Event

A dropstone‐bearing, Middle Permian to Early Triassic peri‐glacial sedimentary unit was first discovered from the Khangai–Khentei Belt in Mongolia, Central Asian Orogenic Belt. The unit, Urmegtei Formation, is assumed to cover the early Carboniferous Khangai–Khentei accretionary complex, and is an upward‐fining sequence, consisting of conglomerates, sandstones, and varved sandstone and mudstone beds with granite dropstones in ascending order. The formation was cut by a felsic dike, and was deformed and metamorphosed together with the felsic dike. An undeformed porphyritic granite batholith finally cut all the deformed and metamorphosed rocks. LA‐ICP‐MS, U–Pb zircon dating has revealed the following ²⁰⁶Pb/²³⁸U weighted mean igneous ages: (i) a granite dropstone in the Urmegtei Formation is 273 ± 5 Ma (Kungurian of Early Permian); (ii) the deformed felsic dike is 247 ± 4 Ma (Olenekian of Early Triassic); and (iii) the undeformed granite batholith is 218 ± 9 Ma (Carnian of Late Triassic). From these data, the age of sedimentation of the Urmegtei Formation is constrained between the Kungurian and the Olenekian (273–247 Ma), and the age of deformation and metamorphism is constrained between the Olenekian and the Carnian (247–218 Ma). In Permian and Triassic times, the global climate was in a warming trend from the Serpukhovian (early Late Carboniferous) to the Kungurian long and severe cool mode (328–271 Ma) to the Roadian to Bajocian (Middle Jurassic) warm mode (271–168 Ma), with an interruption with the Capitanian Kamura cooling event (266–260 Ma). The dropstone‐bearing strata of the Urmegtei Formation, together with the glacier‐related deposits in the Verkhoyansk, Kolyma, and Omolon areas of northeastern Siberia (said to be of Middle to Late Permian age), must be products of the Capitanian cooling event. Although further study is needed, the dropstone‐bearing strata we found can be explained in two ways: (i) the Urmegtei Formation is an autochthonous formation indicating a short‐term expansion of land glacier to the central part of Siberia in Capitanian age; or (ii) the Urmegtei Formation was deposited in or around a limited ice‐covered continent in northeast Siberia in the Capitanian and was displaced to the present position by the Carnian.     

安徽寿县新元古界一个滑塌-滑脱软沉积物变形复合构造的发现及地质初探

安徽寿县新元古界四十里长山组底部粉砂岩层中发现了一个滑塌-滑脱软沉积物变形复合构造,剖面观察显示,该沉积变形构造经历了滑塌变形、滑脱变形、震动液化变形等3次以上的变形过程,很好地保存了原始沉积面貌和沉积变形特征;分析该沉积变形构造的成因,发现变形构造属于软沉积物变形构造类型,形成于浅海陆棚边缘斜坡相带。引起软沉积物变形的动力是地震事件产生的多次震动波作用,造成软沉积物滑塌、滑脱褶皱、震动液化泄水等变形作用,形成了具有复杂变形特征的软沉积物变形复合体,是一次地震事件多次地震活动的沉积记录。     

Ping Jian (1964–2015)

ABSTRACT

Metasedimentary rocks from the El Triunfo Complex (Jocote Unit) in the southern Chiapas Massif (SE México) are constituted mainly by sillimanite-rich micaschist, locally intercalated with marble and calc-silicate rocks. Mafic rocks (now amphibolite) intruded the sequence prior to deformation and folding. Peak metamorphic conditions are estimated by geothermobaromerty at ~6.0 kbar and ~650ºC. The timing of the metamorphic event is dated by LA-MC-ICPMS analysis on zircon rims at 438⁺²³/_–12 Ma. Furthermore, detrital zircon grains yield mainly Stenian–Tonian and minor early Mesoproterozoic ages, indicating provenance from Grenville-type orogens (such as Oaxaquia) and some older cratonic sources. The ⁸⁷Sr/⁸⁶Sr values of 0.70775–0.70777 and the δ¹³C values from +1.9‰ to +2.7‰ in associated calcite marble define the time of deposition between 600 and 580 Ma. Geochemical markers from metapelite samples (such as La/Th > 3.94, La/Sc > 3.72, Th/U > 8.19, Th/Co > 0.42 and CIA = 74 to 83), as well as Sm–Nd isotope data (εNd_i = ?8.1 to ?4.0, TDM_(Nd) = 1.65–1.32 Ga) suggest weathering of Mesoproterozoic felsic rocks during temperate to warm climate. Furthermore, Zr/Sc values (9.1–21.0), chondrite-normalized REE patterns [La/Yb]_N = 10.3–23.3, Eu/Eu* < 0.64), and ΔHf values (1.98–10.02) are indicative of pelagic and zircon-depleted sediments of a passive margin. The results suggest that the Jocote Unit was deposited during the opening of the Eastern Iapetus Ocean in the Ediacaran Period. This is the first evidence for Rodinia breakup in southern México. Besides that, the Ordovician tectonothermal event is probably related to compression during subduction and accretion along the western margin of Gondwana.     

Late Jurassic terrane collision in the northwestern margin of Gondwana (Cajamarca Complex,eastern flank of the Central Cordillera,Colombia)

Medium-grade metabasites and metapelites from the Cajamarca Complex (Central Cordillera of Colombia) are in fault contact with the Jurassic Ibague batholith and show a penetrative foliation, locally mylonitic, suggesting intense dynamic–thermal metamorphism. The amphibolites are composed of calcic amphibole + epidote + plagioclase + quartz plus rutile + titanite + apatite + carbonate as accessory phases. Chlorite and albite appear as retrograde replacements. The metapelites are mainly composed of phengite + quartz + garnet + chlorite, plus epidote + albite + apatite + titanite + haematite as accessory phases. Bulk geochemistry of the amphibolites indicates basaltic protoliths with a mid-ocean ridge basalt (MORB) signature, although enrichment in the mobile large-ion lithophile elements compared to MORB suggests pre- and/or syn-metamorphic alteration by fluids. Peak pressure–temperature determinations for both types of rocks are similar, ranging 550–580°C and 8 kbar (approximately 26 km depth and an apparent geothermal gradient of 22°C/km). ⁴⁰Ar-³⁹Ar dating of amphibole from two amphibolite samples and one phengitic mica from a pelitic schist yielded plateau ages of 146.5 ± 1.1 Ma and 157.8 ± 0.6 Ma, and 157.5 ± 0.4 Ma, respectively. These Late Jurassic ages contrast with previously published (Permian)Triassic ages of metamorphism in the Cajamarca Complex. Taken together, our data indicate tectonic-driven burial of oceanic supracrustal sequences down to mid-crustal depths during Late Jurassic times and are best explained as the result of terrane collision-related metamorphism and deformation in a fore-arc/volcanic-arc environment of the active western margin of Gondwana rather than as a result of Jurassic thermal–metamorphic resetting of a (Permian)Triassic metamorphic sequence during intrusion of the Jurassic Ibague batholith. Our results represent the first report of Jurassic terrane collision tectonics involving supracrustal oceanic rocks in the northwestern margin of Gondwana in Colombia.     

Tsunami backwash deposits with Chicxulub impact ejecta and dinosaur remains from the Cretaceous–Palaeogene boundary in the La Popa Basin,Mexico

The La Popa Basin in north‐eastern Mexico features outstanding, continuous three‐dimensional exposures of the Cretaceous–Palaeogene boundary event deposit in shallow shelf environments pierced by salt stocks. In the area to the south‐east of the El Papalote diapir, the Cretaceous–Palaeogene deposit consists of two superimposed sedimentary units and erosively overlies upper Maastrichtian sand‐siltstones with soft‐sediment deformation and liquefaction structures. The basal unit 1 is an up to 8 m thick chaotic, carbonate‐rich bed that discontinuously fills incised gutters and channels. Besides abundant silicic and carbonate ejecta spherules from the Chicxulub impact, unit 1 includes large sandstone boulders and abundant shallow‐water debris (for example, mud clasts, algae, bivalve shells, gastropod shells and vertebrate remains). Unit 1 is conformably overlain by unit 2. Distal to the diapir, unit 2 consists of a centimetre to decimetre‐thick conglomeratic, coarse bioclast and spherule‐bearing sandstone bed. Closer to the diapir, unit 2 becomes a metre‐thick series of four to eight conglomeratic to fine‐grained graded sandstone beds rich in shell debris and ejecta spherules. Unit 2 is conformably overlain by structureless to parallel laminated sandstone beds that may mark the return to the pre‐event depositional regime. The sedimentary characteristics of the Cretaceous–Palaeogene deposit, including its erosive base, its sheet‐like geometry, the presence of multiple, graded beds, evidence for upper flow regime conditions and the absence of bioturbation, support an origin by a short‐term multiphase depositional event. The occurrence of soft‐sediment deformation structures (for example, liquefaction) below the Cretaceous–Palaeogene deposit suggests that earthquakes were the first to occur at La Popa. Then, shelf collapse and strong backflow from the first tsunami waves may have triggered erosion and deposition by violent ejecta‐rich hyperconcentrated density flows (unit 1). Subsequently, a series of concentrated density flows resulting from tsunami backwash surges may have deposited the multiple‐graded bedding structures of unit 2. The specific depositional sequence and the Fe‐Mg‐rich as well as Si‐K‐rich composition of the ejecta spherules both provide a critical link to the well‐known deep marine Cretaceous–Palaeogene boundary sites in the adjacent Burgos basin in north‐eastern Mexico. Moreover, the pulse‐like input of Chicxulub ejecta material at the base of the event deposit allows for correlation with other Cretaceous–Palaeogene boundary sites in the Gulf of Mexico and the Atlantic, as well as in Central and Northern America. The presence of diverse dinosaur and mosasur bones and teeth in the event deposit is the first observation of such remains together with Chicxulub ejecta material. These findings indicate that dinosaurs lived in the area during the latest Maastrichtian and suggest that the tsunami waves not only eroded deltas and estuaries but the coastal plain as well.     

碰撞与花岗岩——碰撞是构造事件,不是构造环境

碰撞与花岗岩的关系是学术界关心的问题,但是,当前在碰撞与花岗岩关系的研究中存在许多误区.本文认为,碰撞是地壳浅部的构造事件,不属于构造环境范畴.碰撞本身不产生花岗岩,花岗岩形成需要热,热主要来自地幔,是来自地幔的热使下地壳底部熔融才形成了花岗岩.碰撞和碰撞后花岗岩地球化学性质不同,原因与碰撞或碰撞后无关,而与碰撞导致的地壳厚度变化有关.碰撞不是构造环境,现今所用的花岗岩构造环境判别图如果包含有碰撞的内容全部是错误的.     

福建罗源湾潮滩沉积过程对人类活动和台风事件的响应

潮滩地区是人类活动和台风事件影响最为显著的地区之一,利用潮滩沉积记录可以提取影响区域重大人类活动和台风事件的历史变化。罗源湾潮滩中上部盐沼和光滩柱状沉积物粒度分析结果显示,罗源湾潮滩沉积物以细颗粒为主,盐沼内部和潮滩中上部光滩柱状沉积物平均粒径分别为5.41~45.00 μm和5.4~68.82 μm,但在不同深度出现多次沉积物变粗的现象;利用210Pb测试计算得到的沉积速率分别为1.96 cm/a和1.23 cm/a;近年来的围填海活动导致潮间带地区沉积速率显著增大,互花米草的引种也进一步导致潮间带上部沉积速率增大。台风的强烈影响使得潮间带沉积出现间断,在沉积间断界面出现沉积物粒度变粗的现象,由此推断了近170年来显著影响罗源湾地区的台风事件沉积层的位置,并由此计算了不同时段的沉积速率。根据柱状沉积物粒度分析结果,不仅可以反演近百年来的对罗源湾产生重要影响的台风事件,也可以反演20世纪50年代以来罗源湾经历的几次较大规模的人类活动。     

基于Matlab的最小相位滤波在处理影区地震中的初步研究

研究基于Matlab的最小相位滤波器在处理影区地震中的应用。使用共轭倒序法设计滤波器对合肥地震台记录的中国台湾影区地震进行滤波处理,有效抑制地动噪声干扰,突出地震信号特征,为地震分析提供较好的帮助。