Magnetic fabric study of the SE Rhenohercynian Zone (Bohemian Massif): Implications for dynamics of the Paleozoic accretionary wedge

The progressive deformation recorded in the magnetic fabric of sedimentary rocks was studied in the SE Rhenohercynian Zone (RHZ), eastern margin of the Bohemian Massif, Czech Republic. Almost 800 oriented samples of the Lower Carboniferous mudstones and graywackes were collected from the SSE part of the Czech RHZ, so-called the Drahany Upland. The anisotropy of magnetic susceptibility (AMS) is predominantly controlled by the preferred orientation of paramagnetic phyllosilicates, mainly iron-bearing chlorites. A regional distribution of the magnetic fabric within the Drahany Upland revealed an increasing deformation from the SSE to the NNW. In the SE, the magnetic fabric is bedding-parallel with magnetic lineation scattered in the bedding plane or trending N–S to NNE–SSW. Further to the NW, the magnetic foliation rotates from the bedding-parallel orientation to the orientation parallel to the evolving cleavage. This rotation is accompanied by a decrease of the anisotropy degree and the prolate nature of the anisotropy ellipsoids. The magnetic lineation is parallel to the strike of the bedding, bedding/cleavage intersection, pencil structure or the fold axes on a regional scale. In the NW part of the Drahany Upland, the magnetic foliation becomes parallel to the cleavage accompanied by an increase of the anisotropy degree and the oblate nature of the anisotropy ellipsoids. The increasing trend of deformation corresponds to the SSE–NNW increase in the degree of anchimetamorphism; both trends being oblique to the main lithostratigraphic formations as typically observed in the sedimentary rocks of the accretionary wedges. The SSE–NNW increase in deformation and anchimetamorphism continues to the Nízký Jeseník Mts., representing the northern part of the same accretionary wedge. The kinematics of deformation could not be unambiguously assessed. The observed magnetic fabric may reflect either lateral shortening or horizontal simple shear or a combination of both mechanisms. Regarding the subduction process, it seems that the sedimentary sequences of the Drahany Upland were subducted, partly offscraped and accreted frontally or partly underplated as opposed to the Nízký Jeseník Mts. where some return flow must have occurred.     

Anisotropy of magnetic susceptibility and paleomagnetic studies in relation to the tectonic evolution of the Miocene–Pleistocene accretionary sequence in the Boso and Miura Peninsulas, central Japan

Anisotropy of magnetic susceptibility (AMS) and paleomagnetic methods have been applied on the middle Miocene–Pleistocene sedimentary sequence in the Boso and Miura Peninsulas of central Japan in order to identify the invisible regional deformation sense as well as the intensity of deformation of sediments. The southern sequences of the two peninsulas were subjected to syn-sedimentary deformation of folding and faulting generated in compressional tectonics. A previous result of the AMS experiment on the sequences shows a development of a strong magnetic lineation. Thus, it is conceivable that the lineation had to be generated during the process of deformation, and in a direction perpendicular to the shortening. However, the orientation of the magnetic lineations is inconsistent among the different tectonic domains in the southern sequence. The paleomagnetic declination in each domain reveals a clockwise rotation in various degrees. Reconstructed directions of the magnetic lineations show a consistent pattern in the east–west direction, suggesting that the sedimentary sequence was subjected to a north-southward compression. In contrast, the compressive direction of the sediment cover on the Pliocene–Pleistocene sequence reveals a northwest direction. Our results suggest that the Philippine Sea Plate had been subducting northward during the middle Miocene–Pliocene, and changed its direction during the Pliocene.     

台湾西部前陆盆地的构造格局和演化规律

基于对台湾岛、台湾海峡和周边海域(南海、东海)的盆山系统、相应动力学机制、构造单元以及其他诸多造山带和毗邻前陆盆地的研究,提出了台湾西部前陆盆地演化的4阶段模式,即:①增生楔发育-单向物源阶段;②挠曲响应-双向物源阶段;③前陆盆地系统形成阶段;④造山带坍塌-前陆衰退阶段。并对各阶段的构造特征、沉积特征和物源供给方式的差异给予了探讨,认为在增生楔发育-单向物源阶段,尽管增生楔业已推覆至陆缘层序之上,但盆地依然呈被动陆缘的外廓特征,以大陆方向为盆地的唯一物源;在挠曲响应-双向物源阶段,盆地开始明显出现挠曲响应,增生楔开始具备并逐步加大向盆地供应沉积物质的能力,但依然以大陆方向为主要物源方向;在前陆盆地系统形成阶段,前渊和前隆区逐步形成,在来自唯一物源、后陆方向造山带物质的快速充填下,前陆期层序快速向前隆方向超覆,形成典型的前陆楔形体;在造山带坍塌-前陆衰退阶段,伸展作用出现并快速扩展,前渊抬升,前陆层序遭受剥蚀,火山作用重新出现。并探讨了该模式是否具有周缘前陆盆地的普遍适用性。     

宁夏中南部中新世构造活动的地质证据及其意义

通过区域性新生界的对比及不整合面分布的研究,发现宁夏地区中新世曾发生过两次构造运动,一次发生于早中新世,另一次发生于中中新世晚期,但以后者明显,分布也最广。其中前者集中在六盘山以西地区,后者分布在查汗布拉格—三关口—青铜峡—固原(查-固)断裂沿线。宁夏中北部大部分红柳沟组是在受中新世构造运动影响的环境下沉积的。发生于早中新世的构造运动集中体现在六盘山以西的地区和甘肃部分地区,该期运动使渐新统清水营组变形,由于这些现象多发生于海原断裂附近,因此推测在早中新世海原断裂就可能已活动了。而发生于中中新世晚期的运动体现在如下几个方面,在贺兰山中南段西侧发现了中中新世晚期的逆冲推覆活动,堆积了厚度较大的同构造沉积(红柳沟组);在查-固断裂沿线发现了由于中中新世晚期的构造活动而形成的红柳沟组与清水营组之间的角度不整合面,这些说明在该期青藏高原的变形前锋已到达查-固断裂,也就是鄂尔多斯西缘地区,早新生代盆地因此解体;与此同时盆地内部红柳沟组向上逐渐变粗,也说明中中新世晚期较强烈的构造运动已影响到了该区,香山地区开始隆起并分割了早先的盆地,使之成为背驮盆地。宁夏中南部并入到青藏高原东北缘逆冲构造楔之中,该逆冲构造楔中新世向北东扩展的原因可能是由于高原北部在同期显著的隆升运动使得逆冲楔顶角超过临界值,而向前扩展的,查-固断裂是早期类似于目前海原断裂的青藏高原变形前缘,整个高原东北部的扩展并不是前展式,而是一种没有特定顺序的变形。上述这些现象否定了前人认为宁夏地区第三系的变形发生在上新世末期—更新世以后的观点。     

论楔入造山作用──以龙门山造山带为例

本文以龙门山造山带为例，根据对阿尔泰—台湾地学断面地质学、地球物理学和地球化学等的综合研究，重点讨论了龙门山造山带岩石圈楔状构造的几何结构样式，探讨了碰撞后的陆内造山过程、造山模式及其科学意义。     

Thermal structure of the Barbados accretionary complex

Finite element modeling of the thermal structure within the Barbados subduction complex is carried out. Kinematics of the sediments inside the complex are computed from a viscous model with inhomogeneous viscosity and the effect of gravity. The model yields an uplift rate compatible with observational data. Advective heat transfer affects the heat flow across the complex. Imbricated thrust faulting further reduces the heat flow across the slope. These mechanisms predict an arcward decrease of heat flow on the lower slope, followed by an increase of heat flow approaching the ridge, little change in heat flow at the forearc basin, and a significant increase of heat flow near the volcanic are, in agreement with the existing observations.     

Kamafugitic diatremes: facies characterisation and genesis—examples from the Goiás Alkaline Province, Brazil

This paper describes the internal organisation of two diatremes (Águas Emendadas and Neuzinha) and one small breccia-filled conduit (Tigre) in the central portion of the Late Cretaceous Goiás Alkaline Province (GAP), central Brazil, and explores the criteria for facies recognition. The GAP kamafugitic diatremes are emplaced into Carboniferous sandstones of the Aquidauana Formation, at the northern margin of the Paraná Basin. They are usually elliptical structures, not longer than 900 m, filled with breccia and partially covered by thin kamafugitic to basanitic lava flows. The breccias are dominated by juvenile pyroclasts, with subordinate amounts of cognate fragments and xenoliths. In addition to variations in ash and lapilli proportions, juvenile fragment types may be used to discriminate genetic processes and the corresponding pyroclastic deposits.

An extensive field, textural and compositional dataset was analysed by multivariate statistical techniques. Combined with field observations, this allowed us to define a set of facies for kamafugitic diatremes, and, more importantly, to understand the internal structure of the studied bodies and to cross-correlate them. Seven distinct facies were recognised. The Fluidised Conduit Facies (FCF) represents high-energy, strongly fluidised but only moderately fragmented systems. It occurs in a confined environment, and is typical of deeper parts of the conduit, before the actual diatreme level is reached by the ascending fluidised magma. Large amounts of spinning droplets are formed within this region. The Fluidised Conduit–Diatreme Facies (FCDF) is characteristic of intermediate depths in the conduit, where highly fluidised and highly fragmented systems produce large amounts of ash. Spinning droplets decrease in abundance, ordinary juvenile fragments become very common, and xenoliths from the country rock in the immediate vicinity of the diatreme are present. The Fluidised Fragmented Facies (FFF) and the Magmatic Fluidised Facies (MFF) produce very heterogeneous deposits that dominate the shallower part of the system, making up most of the diatreme-filling materials. The Fluidised Fragmented Facies can be distinguished by much higher degrees of fluidisation, fragmentation and system energy. It occupies the internal part of the diatreme and is characterised by the common presence of tuff pockets, tuff fragments, and accretionary and armoured lapilli. The Magmatic Fluidised Facies typically occupies the outer portion of the diatreme and can be distinguished from the Fluidised Fragmented Facies by the dominance of lapilli over ash and by the presence of abundant wrapped fragments. The Magmatic Facies (MF) and the Coherent Magmatic Facies (CMF) are volumetrically subordinate and represent late stages, when less fluidised and less fragmented material, or even coherent magma erupts relatively passively, in the aftermath of the main explosive stage that generated the diatreme. The Border Facies is defined by the increased abundance of material from the immediate country rock. At Águas Emendadas and Neuzinha this facies is marked by the presence of fragments of peperite-like rock, formed by the interaction of the fluidised magma with friable sandstone.     

Enregistrement de la dernière remontée du niveau marin dans l'architecture interne d'une vallée incisée : le pertuis Breton (Charente-Maritime)

Recent very high-resolution seismic profiles ground-truthed by vibrocores allow us to evidence an atypical incised valley fill in a drowned valley segment, the ‘Pertuis Breton’ (Bay of Biscay, France). The sedimentary valley-fill architecture mainly includes five superimposed progradational wedges composed by marine sands. Sandbodies show a landward migration of their depocentres upward and are topped by almost flat unconformities extended by submarine terraces. This sedimentary infill pattern is similar to backstepping wedges, described on continental shelfs. It suggests that this valley fill records sea-level rise during the last transgression. To cite this article: N. Weber et al., C. R. Geoscience 336 (2004).     

Tectonic wedging along the rear of the offshore Taiwan accretionary prism

The structural geometry, kinematics and density structure along the rear of the offshore Taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling. Deformation between the offshore prism and forearc basin at the point of incipient collision, and southward into the region of subduction, has been interpreted as a tectonic wedge, similar to those observed along the front of mountain ranges. This tectonic wedge is bounded by an east-dipping roof thrust and a blind, west-dipping floor thrust. An east-dipping sequence of forearc-basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge. Section restoration of the roof sequence yields an estimate of 4 km of shortening, which is small compared with that inferred in the collision area to the north, based on the variation in distance between the apex of the prism and the island arc.Previous studies propose that either high-angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the Central Range in the collision zone on land. To better constrain the initial crustal configuration, we tested 350 crustal models to fit the free-air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land. The gravity anomaly, observed in the region of subduction (20.2°N), can be modeled with the arc basement forming a trenchward-dipping backstop that is overlain by materials with densities in the range of sedimentary rocks. Near the point of incipient collision (20.9°N), however, the free-air gravity anomaly over the rear of the prism is approximately 40 mgal higher, compared with the region of subduction, and requires a significant component of high density crustal rocks within the tectonic wedge. These results suggest that the forearc basement may be deformed along the rear of the prism, associated with the onset of collision, but not in the subduction region further to the south.     

The distribution of solute processes on an acid hillslope and the delivery of solutes to a stream: I. Exchangeable bases

This paper aims to identify the spatial distribution of exchangeable base cations in soils on an acid hillslope and to investigate possible cation release processes from slope soils to the stream. The basic assumption underlying this research is that the amount of exchangeable cations in soils reflects the nutrient stores and cation leaching processes across the slope where vegetation and parent materials are similar. The distribution of exchangeable Ca²⁺, Mg²⁺, K⁺ and Na⁺ has been investigated on a three-dimensional hillslope on the Quantock Hills, Somerset, UK. A two-way ANOVA shows that soil depth is predominant in explaining the total variance of exchangeable bases, despite the steep slope gradient and clear podzolic catena development. Major nutrient base cations, such as Ca²⁺, Mg²⁺ and K⁺, display homogeneous topsoil storage right across the slope. This spatial pattern may indicate that the spatial distribution of major nutrient cations is tightly controlled by the soil–vegetation system in nutrient-poor heathland environments. Na⁺ is an exception to this vegetation-controlled spatial distribution, because of its small involvement in the soil–vegetation and soil exchangeable systems. In subsurface soils, cations liberated from the soil–vegetation system are subject to redistribution over the slope according to the hydrological flowpaths operating on the slope, with some eventually released into the stream. The saturated wedge developed at the base of the slope plays a key role in the storage and release processes of base cations from slope soils to the stream. Ca²⁺, Mg²⁺ and Na⁺ carried by throughflow are stored in the saturated wedge and gradually released into the stream at times of high flow. K⁺, however, shows an apparently different spatial behaviour, being deficient in the saturated wedge. Copyright © 1999 John Wiley & Sons, Ltd.