Discrete-element modelling of detachment folding

A two‐dimensional, discrete‐element modelling technique is used to investigate the initiation and growth of detachment folds in sedimentary rocks above a weak décollement level. The model depicts the sedimentary rocks as an assemblage of spheres that obey Newton's equations of motion and that interact with elastic forces under the influence of gravity. Faulting or fracturing between neighbouring elements is represented by a transition from repulsive–attractive forces to solely repulsive forces. The sedimentary sequence is mechanically heterogeneous, consisting of intercalated layers of markedly different strengths and thicknesses. The interlayering of weak and strong layers within the sedimentary rocks promotes the localization of flexural flow deformation within the weak layers. Even with simple displacement boundary conditions, and straightforward interlayering of weak and strong layers, the structural geometries that develop are complex, with a combination of box, lift‐off and disharmonic detachment fold styles forming above the décollement. In detail, it is found that the modelled folds grow by both limb rotation and limb lengthening. The combination of these two mechanisms results in uplift patterns above the folds that are difficult, or misleading, to interpret in terms of simple kinematic models. Comparison of modelling results with natural examples and with kinematic models highlights the complexities of structural interpretation in such settings.     

Testing long-term patterns of basin sedimentation by detrital zircon geochronology, Centralian Superbasin, Australia

Detrital zircon geochronology of Neoproterozoic to Devonian sedimentary rocks from the Georgina and Amadeus basins has been used to track changes in provenance that reflect the development and inversion of the former Australian Superbasin. Through much of the Neoproterozoic, sediments appear to have been predominantly derived from local sources in the Arunta and Musgrave inliers. Close similarities between the detrital age signatures of late Neoproterozoic sedimentary rocks in the two basins suggests that they were contiguous at this time. A dominant population of 1.2–1.0 Ga zircon in Early Cambrian sediments of the Amadeus Basin reflects the uplift of the Musgrave Inlier during the Petermann Orogeny between 560 and 520 Ma, which shed a large volume of detritus northwards into the Amadeus Basin. Early Cambrian sedimentary rocks in the Georgina Basin have a much smaller proportion of 1.2–1.0 Ga detritus, possibly due to the formation of sub‐basins along the northern margin of the Amadeus Basin which might have acted as a barrier to sediment transfer. An influx of 0.6–0.5 Ga zircon towards the end of the Cambrian coincides with the transgression of the Larapintine Sea across central Australia, possibly as a result of intracratonic rifting. Detrital zircon age spectra of sedimentary rocks deposited within this epicontinental sea are very similar to those of coeval sedimentary rocks from the Pacific Gondwana margin, implying that sediment was transported into central Australia from the eastern continental margin. The remarkably consistent ‘Pacific Gondwana’ signature of Cambro‐Ordovician sediments in central and eastern Australia reflects a distal source, possibly from east Antarctica or the East African Orogen. The peak of the marine incursion into central Australia in the early to mid Ordovician coincides with granulite‐facies metamorphism at mid‐crustal depths between the Amadeus and Georgina basins (the Larapinta Event). The presence of the epicontinental sea, the relative lack of a local basement zircon component in Cambro‐Ordovician sedimentary rocks and their maturity suggest that metamorphism was not accompanied by mountain building, consistent with an extensional or transtensional setting for this tectonism. Sediments deposited at ～435–405 and ～365 Ma during the Alice Springs Orogeny have detrital age signatures similar to those of Cambro‐Ordovician sedimentary rocks, reflecting uplift and reworking of the older succession into narrow foreland basins adjacent to the orogen.     

The elemental geochemical characteristics of Late Quaternary rock core from Yanwo Lake in the Great Wall Station area， King George Island， Southwest Antarctica

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微量元素分析在判别沉积环境中的应用 ——以柴达木盆地西部锶矿区富锶地层为例

沉积岩中的微量元素对沉积环境水介质的变化较为敏感,是研究沉积物沉积时古气候、古环境的有效手段。本文选取了对沉积环境介质较为敏感的（Sr）、钡（Ba）、铜（Cu）、铀（U）、钒（V)、镍（Ni）、钍（Th）等元素作为判别指标,对柴达木盆地西部中新统油砂山组、狮子沟组和更新统七个泉组三套富锶岩层的古气候和古沉积环境特征进行了研究。结果表明：柴达木盆地西部从油砂山组到七个泉组沉积时总体上为干旱的气候环境;湖盆水体的盐度较高,表现为咸水环境,且从油砂山组到七个泉组随着地层变新,盐度有逐渐增大的趋势;油砂山组、狮子沟组和七个泉组沉积时湖盆水体均表现为中等分层,湖水循环较为顺畅的氧化环境。     

Fractal pattern integration for mineral potential estimation

Concepts of fractal/multifractal dimensions and fractal measure were used to derive the prior and posterior probabilities that a small unit cell on a geological map contains one or more mineral deposits. This has led to a new version of the weights of evidence technique which is proposed for integrating spatial datasets that exhibit nonfractal and fractal patterns to predict mineral potential. The method is demonstrated with a case study of gold mineral potential estimation in the Iskut River area, northwestern British Columbia. Several geological, geophysical, and geochemical patterns (Paleozoic-Mesozoic sedimentary and volcanic clastic rocks; buffer zones around the contacts between sedimentary rocks and Mesozoic intrusive rocks; a linear magnetic anomaly; and geochemical anomalies for Au and associated elements in stream sediments) were integrated with the gold mineral occurrences which have fractal and multifractal properties with a box-counting dimension of 1.335±0.077 and cluster dimension of 1.219±0.037.     

Late Palaeogene palynoflora from Point Hennequin of the Admiralty Bay , King George Island, Antarctica with reference to its stratigraphical significance

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Example of a supradetachment basin within a pull-apart tectonic setting: Mormon Point, Death Valley, California

The geological features now exposed at Mormon Point, Death Valley, reveal processes of extension that continue to be active, but are concealed beneath the east side of Death Valley. Late Cenozoic sedimentary rocks at Mormon Point crop out in the hangingwall of the Mormon Point low-angle normal fault zone, a fault zone that formed within a releasing bend of the oblique-slip (right-normal slip) fault zone along the east side of Death Valley. The late Cenozoic sedimentary rocks were part of the valley when the low-angle fault zone was active, but during late Quaternary time they became part of the Black Mountains block and were uplifted. Rocks and structures exposed at Mormon Point are an example of the types of features developed in a releasing bend along the margins of a major pull-apart structure, and in this example they are very similar to features associated with regional detachment faults. The oldest sedimentary rocks in the hangingwall of the Mormon Point low-angle fault zone dip steeply to moderately east or north-east and were faulted and rotated in an extensional kinematic environment different from that recorded by rocks and structures associated with younger rocks in the hangingwall. Some of the younger parts of the late Cenozoic sedimentary rocks were deposited, faulted and rotated during movement on the Mormon Point low-angle normal fault. Progressively, strata are less faulted and less rotated. The Mormon Point low-angle normal fault has an irregular fault surface whose segments define intersections that plunge 18°-30°, N10°-40°W, with a maximum of 22°, N22°W that we interpret to be the general direction of slip. Thus, even though Death Valley trends north, movement on the faults responsible for its formation was at least locally north-northwest. Gouge and disrupted conglomerates along the faults are interpreted to have formed either as adjustments to accommodate space problems at the corners of blocks or along faults that bounded blocks during their displacement and rotation. The younger units of the late Cenozoic sedimentary rock sequence and the geomorphic surfaces developed on them are rarely faulted, not rotated, and overlap the Mormon Point low-angle faults. Active faults cut Holocene alluvium north of the late Cenozoic rocks and form the present boundary between Mormon Point and the Black Mountains. The distribution of active faults defines a releasing bend that mimics the older releasing bend formed by the Mormon Point low-angle fault zone. Rocks and structures similar to those exposed above the Mormon Point low-angle fault zone are probably forming today beneath the east side of Death Valley north-west of Mormon Point.     

微量元素分析在判别沉积环境中的应用——以柴达木盆地西部锶矿区富锶地层为例

沉积岩中的微量元素对沉积环境水介质的变化较为敏感,是研究沉积物沉积时古气候、古环境的有效手段。选取了对沉积环境介质较为敏感的锶(Sr)、钡(Ba)、铜(Cu)、铀(U)、钒(V)、镍(Ni)、钍(Th)等元素作为判别指标,对柴达木盆地西部上新统油砂山组、狮子沟组和更新统七个泉组三套富锶岩层的古气候和古沉积环境特征进行了研究。结果表明,柴达木盆地西部从油砂山组到七个泉组沉积时总体上为干旱的气候环境;湖盆水体的盐度较高,表现为咸水环境,且从油砂山组到七个泉组随着地层变新,盐度有逐渐增大的趋势;油砂山组、狮子沟组和七个泉组沉积时湖盆水体均表现为中等分层、湖水循环较为顺畅的氧化环境。     

Constraining recycled detritus in quartz‐rich sandstones: Insights from a multi‐proxy provenance study of the Mid‐Carboniferous,Clare Basin,western Ireland

Quartz‐rich sandstones can be produced through multiple sedimentary processes, potentially acting in combination, such as extensive sedimentary recycling or intense chemical weathering. Determining the provenance of such sedimentary rocks can be challenging due to low amounts of accessory minerals, the fact that the primary mineralogy may have been altered during transport, storage or burial and difficulties in the recognition of polycyclic components. This study uses zircon and apatite U‐Pb geochronology, apatite trace elements, zircon‐tourmaline‐rutile indices and petrographic observations to investigate the sedimentary history of mineralogically mature mid‐Carboniferous sandstones of the Tullig Cyclothem, Clare Basin, western Ireland. The provenance data show that the sandstones have been dominantly and ultimately sourced from three basement terranes: older Laurentian‐ associated rocks (ca. 900–2500 Ma) which lay to the north of the basin, peri‐Gondwanan terranes (ca. 500–700 Ma) to the south and igneous intrusive rocks associated with the Caledonian Orogenic Cycle (ca. 380–500 Ma). However, the multi‐proxy approach also helps constrain the sedimentary history and suggests that not all grain populations were derived directly from their original source. Grains with a Laurentian or a Caledonian affinity have likely been recycled through Devonian basins to the south. Grains with a peri‐Gondwanan affinity appear to be first cycle and are potentially derived from south/southwest of the basin. Taken as a whole, these data are consistent with input into the basin from the south and southwest, with the reworking of older sedimentary rocks, rather than intensive first‐cycle chemical weathering, likely explaining the compositional maturity of the sandstones. This study highlights the need for a multi‐proxy provenance approach to constrain sedimentary recycling, particularly in compositionally mature sandstones, as the use of zircon geochronology alone would have led to erroneous provenance interpretations. Zircon, together with U‐Pb geochronology from more labile phases such as apatite, can help distinguish first‐cycle versus polycyclic detritus.     

Aggregation of Nanogold Particles in the Environment

Nanogold exists widespread in the environment. For example, countless nanogold particles appear in weathered rocks due to chemical decomposition of mineral-concentrators (e.g., pyrite, arsenopyrite, pyrrhotite, and magnetite). Nanogold is usually represented by isolated particles dispersed in minerals and sedimentary rocks, but in certain conditions, Au nanoparticles are able to merge with each other and compose simple or complex aggregates. The mechanisms of aggregation are related to specific geochemical situations in the environment. In addition, superfluous surface energy of metal nanoparticles and their chemical activity essentially contribute to aggregation. There are two main ways of accumulation, namely adsorption on surface of minerals (mainly placer Au grains) and gradual consolidation of separate Au nanoparticles due to natural amalgamation. Aggregation of Au nanoparticles is a natural mechanism of accumulation in sedimentary rocks, which may be reproduced in nanotechnologies.     

昌都—兰坪思茅盆地含盐系地层地球化学特征及物源属性探讨

碎屑岩的地球化学特征对应于特定的源区和构造环境,本文利用主微量元素对昌都盆地的早白垩纪景星组地层进行了地球化学分析,并与兰坪思茅盆地的早白垩纪地层进行了物源对比研究。岩相学研究显示景星组沉积岩的成分成熟度较低,其物源以火山岩为主。昌都地区景星组的主量元素地球化学分析显示其源区为石英质沉积岩源区和长英质火成岩源区。相比兰坪思茅地区早白垩纪地层,景星组地层物源的输入比较单一。景星组地层的CIA在59～78之间,平均为67,明显低于扒沙河组地层,表明兰坪思茅盆地的早白垩纪地层的风化强度明显高于昌都盆地。主微量构造环境判别图解显示昌都—兰坪思茅盆地早白垩纪地层的物源构造环境主要为被动大陆边缘与活动大陆边缘,同时具有大陆岛弧的特征。     

乔治王岛地区原始土壤与母岩间化学组成的相互关系

与内生过程相比,表生过程具有显著的区域性特征。本文根据原始土壤与成土母岩间化学组成的相互关系,研究了极地条件下的风化成土过程。分析了乔治王岛土壤中化学元素的原始来源、生物作用及人为影响,推断了原始环境中元素的迁移特征,讨论了极地条件下成土母岩与原始土壤间化学元素丰度的相互关系。     

Late Archean basement in the Bangenhuken Complex of the Nordbreen Nappe, western Ny-Friesland, Svalbard

The rocks of western Ny-Friesland, northern Svalbard, are part of a tectonostratigraphy including four thrust sheets, each composed mainly of orthogneisses overlain by younger metasedimentary rocks. Previous geochronological studies have shown that the orthogneisses are dominated by ca. 1750 Mya granitoids. This study of a quartz-monzonite in one of the thrust sheets, the Nordbreen Nappe, yields a single-zircon U-Pb ion-microprobe age of 2709 Ø 28 My. This is the oldest rock unit so far reported in the Svalbard Caledonides. However, age-determinations on detrital zircons in the metasediments of western Ny-Friesland have shown that Late Archean rocks were prominent sources. The new ages presented here provide the first evidence of a local source for these sedimentary rocks.     

A New Model for Heat Flow in Extensional Basins: Estimating Radiogenic Heat Production

Radiogenic heat production (RHP) represents a significant fraction of surface heat flow, both on cratons and in sedimentary basins. RHP within continental crust—especially the upper crust—is high. RHP at any depth within the crust can be estimated as a function of crustal age. Mantle RHP, in contrast, is always low, contributing at most 1 to 2 mW/m² to total heat flow. Radiogenic heat from any noncrystalline basement that may be present also contributes to total heat flow. RHP from metamorphic rocks is similar to or slightly lower than that from their precursor sedimentary rocks. When extension of the lithosphere occurs—as for example during rifting—the radiogenic contribution of each layer of the lithosphere and noncrystalline basement diminishes in direct proportion to the degree of extension of that layer. Lithospheric RHP today is somewhat less than in the distant past, as a result of radioactive decay. In modeling, RHP can be varied through time by considering the half lives of uranium, thorium, and potassium, and the proportional contribution of each of those elements to total RHP from basement. RHP from sedimentary rocks ranges from low for most evaporites to high for some shales, especially those rich in organic matter. The contribution to total heat flow of radiogenic heat from sediments depends strongly on total sediment thickness, and thus differs through time as subsidence and basin filling occur. RHP can be high for thick clastic sections. RHP in sediments can be calculated using ordinary or spectral gamma-ray logs, or it can be estimated from the lithology.     

粘土矿物的提取、鉴定及其古气候意义——以柴达木盆地怀头他拉剖面为例

沉积物中粘土矿物类型主要有自生粘土矿物和碎屑粘土矿物。其中,自生粘土矿物含量较少,是在沉积环境中形成,可能是沉积再循环或成岩作用的产物,可作为沉积环境某方面的指示标志;而碎屑粘土矿物是母岩风化的产物,受沉积环境影响较小,能够有效示踪物源区化学风化过程,进而反映古气候变化。近年来利用碎屑粘土矿物特征来重建物源区古气候的方法得到了广泛的应用。然而,由于粘土矿物主要存在于<2 μm硅酸盐粘粒中,粘土矿物的提取、鉴定比较困难,且自生粘土矿物易受沉积环境等的影响。因此,在利用粘土矿物重建古气候变化时,需慎重选择粘土矿物的提取方法,并考虑物源和沉积环境变化以及成岩作用等对粘土矿物解释的影响。本文以柴达木盆地东北缘怀头他拉剖面硅酸盐粘土矿物的提取、鉴定及其古环境指示意义为例,介绍粘土矿物的提取及应用,为后期粘土矿物研究提供参考。     

A Review and Evaluation of Specific Heat Capacities of Rocks,Minerals, and Subsurface Fluids. Part 1: Minerals and Nonporous Rocks

Heat capacities of the rocks within a sedimentary basin can significantly influence geothermal gradients if sedimentation or erosion is rapid. This paper provides data on specific heat capacities of minerals and nonporous rocks at 20°C, derives equations for calculating specific heat capacities of minerals and nonporous rocks at temperatures between 0°C and 1200°C, and shows that pressure effects on heat capacities of solids can be neglected. It derives an equation for estimating specific heat capacity of any mineral or nonporous rock as a function of density. Finally, it shows how to calculate the specific heat capacity of any mixture of solid materials. A companion paper discusses specific heat capacities of the fluids in pore spaces of rocks and of fluid-filled porous rocks. The data for minerals and rocks provided herein can be incorporated directly into existing modeling software by users. However, the temperature-dependent equations would have to be incorporated by software developers.     

Discriminating Weathering Degree by Integrating Optical Sensor and SAR Satellite Images for Potential Mapping of Groundwater Resources in Basement Aquifers of Semiarid Regions

Natural Resources Research - Unlike in coastal and sedimentary basins, regional-scale exploration of groundwater resources using only geophysical methods is costlier in consolidated rocks such as...     

粘土矿物在古环境研究中的指示作用

粘土矿物普遍存在于各种类型的沉积物和沉积岩中。作为次生矿物,粘土对环境的变化极为敏感,因而粘土矿物的沉积分异、组合特征、矿物成分及其含量和矿物结晶度都从不同的角度记录了粘土矿物形成过程中各种环境因素的变化。这些对恢复古环境,研究全球变化具有重要的指示意义。     

Geochemistry of Precambrian basic igneous rocks between St. Jonsfjorden and Isfjorden, central western Spitsbergen, Svalbard

Two types of basic igneous rocks have been mapped in the middle Hecla Hoek succession in the area between St. Jonsfjorden and Isfjorden, central western Spitsbergen: the metadiabase-gabbros from the calc-argillo-volcanic formation and the Trollheimen volcanics from the quartzite-shale formation; both formations are older than the Eocambrian tilloid formation. The former has diabasic and gabbroic textures and occurs as thin discontinuous lensoid masses, while the latter exhibits definite extrusive structures with large amounts of pyroclastics and tuffs and relatively small amounts of solid lavas with abundant amygdules. Both basic rocks have been metamorphosed under the greenschist facies conditions, characterized by actinolite-epidote-biotite-albite assemblage.
The metadiabase-gabbros are moderate-high Fe tholeiites and the Trollheimen volcanics are mainly Na-alkaline, accompanied by a small amount of calk alkaline rocks. The immobile minor and trace element contents indicate that the metadiabase-gabbros are oceanic, similar to the MORB, while those of the Trollheimen volcanics show non-oceanic chemical characteristics and occur in shallow marine, shelf-shelfedge sediments.
The calc-argillo-volcanic formation reveals a shallow marine sedimentary environment and this does not conform with the oceanic nature of the metadiabase-gabbros, occurring in the formation. To overcome this disagreement, an idea of changing tectonic position is postulated to bring a shallow marine sedimentary regime onto a mid-oceanic tectonic zone. This hypothesis supports the concept of active consuming margin tectonics of the Svalbard Caledonides and explains, for example, the occurrence of the high-P metamorphic rocks in the western part of the present area.     

南极乔治王岛始新统化石山组沉积火山碎屑岩特征

南极乔治王岛化石山剖面的化石山组主要由沉积火山碎屑岩（包括沉火山角砾岩和沉凝灰岩）组成，上部夹正常沉积岩和两层薄煤层，形成从粗到细的两个沉积旋回。第１旋回的沉火山角砾岩和沉凝灰岩由不同粒级的火山岩岩屑、斜长石晶屑和细火山尘基质组成，火山碎屑物质含量在９０％以上，沉积特征显示从火山碎屑流（？）到低密度流的快速堆积性质。其中第１、２层沉凝灰岩受强烈的沸石化（浊沸石和方沸石）作用，而泥级沉凝灰岩夹层的细火山尘物质多已变成绿泥石－蒙脱石规则混层矿物；因为这种混层矿物也常常被沸石化，故其形成应在沸石化作用之前。Ｘ－衍射分析显示较多钠长石的存在。钠长石和方沸石的钠可能来自中长石被浊沸石化后释放出的Ｎａ＋。第２旋回由沉火山角砾岩、沉凝灰岩夹岩屑长石杂砂岩和薄煤层组成，显示正常沉积物经常被携入沉积盆地。第３、４层的泥级沉凝灰岩以显著的蒙皂石化为特征，沸石化作用不发育；其中的斜长石主要是中长石和（或）拉长石。化石山组泥级沉凝灰岩的微量元素组分及含量与下伏玛瑙滩组顶部安山岩的相当，但第２～３层间有某些元素迁移和富集，这可能与层间热水溶液的活动有关。Ｂ含量特别低（＜１×１０－６），不仅指示背景值低，而且也说明沉积环境与海水没有关