The Misis–Andırın Complex: a Mid-Tertiary melange related to late-stage subduction of the Southern Neotethys in S Turkey

The Mid-Tertiary (Mid-Eocene to earliest Miocene) Misis–Andırın Complex documents tectonic-sedimentary processes affecting the northerly, active margin of the South Tethys (Neotethys) in the easternmost Mediterranean region. Each of three orogenic segments, Misis (in the SW), Andırın (central) and Engizek (in the NE) represent parts of an originally continuous active continental margin. A structurally lower Volcanic-Sedimentary Unit includes Late Cretaceous arc-related extrusives and their Lower Tertiary pelagic cover. This unit is interpreted as an Early Tertiary remnant of the Mesozoic South Tethys. The overlying melange unit is dominated by tectonically brecciated blocks (>100 m across) of Mesozoic neritic limestone that were derived from the Tauride carbonate platform to the north, together with accreted ophiolitic material. The melange matrix comprises polymict debris flows, high- to low-density turbidites and minor hemipelagic sediments.The Misis–Andırın Complex is interpreted as an accretionary prism related to the latest stages of northward subduction of the South Tethys and diachronous continental collision of the Tauride (Eurasian) and Arabian (African) plates during Mid-Eocene to earliest Miocene time. Slivers of Upper Cretaceous oceanic crust and its Early Tertiary pelagic cover were accreted, while blocks of Mesozoic platform carbonates slid from the overriding plate. Tectonic mixing and sedimentary recycling took place within a trench. Subduction culminated in large-scale collapse of the overriding (northern) margin and foundering of vast blocks of neritic carbonate into the trench. A possible cause was rapid roll back of dense downgoing Mesozoic oceanic crust, such that the accretionary wedge taper was extended leading to gravity collapse. Melange formation was terminated by underthrusting of the Arabian plate from the south during earliest Miocene time.Collision was diachronous. In the east (Engizek Range and SE Anatolia) collision generated a Lower Miocene flexural basin infilled with turbidites and a flexural bulge to the south. Miocene turbiditic sediments also covered the former accretionary prism. Further west (Misis Range) the easternmost Mediterranean remained in a pre-collisional setting with northward underthrusting (incipient subduction) along the Cyprus arc. The Lower Miocene basins to the north (Misis and Adana) indicate an extensional (to transtensional) setting. The NE–SW linking segment (Andırın) probably originated as a Mesozoic palaeogeographic offset of the Tauride margin. This was reactivated by strike-slip (and transtension) during Later Tertiary diachronous collision. Related to on-going plate convergence the former accretionary wedge (upper plate) was thrust over the Lower Miocene turbiditic basins in Mid–Late Miocene time. The Plio-Quaternary was dominated by left-lateral strike-slip along the East Anatolian transform fault and also along fault strands cutting the Misis–Andırın Complex.     

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.     

Quantitative description of zircon morphology and its dynamics analysis

The morphology of zircon can be divided into three independent parts: prism habit consisting of 100 and 110, pyramid habit consisting of 211 and 101, and elongation habit consisting of prism and pyramid. Hence, prism index, pyramid index and elongation index are presented in order to describe the morphology of zircon quantitatively. Because these indexes can be used not only for describing the external morphology of zircon, but also for revealing the morphological evolution throughout the history of zircon growth, the theory of crystal growth dynamics can be applied to studying the typomorphism of zircon by means of this method. Project supported by the National Natural Science Foundation of China (Grant Nos. 49672103, 49632080).     

新疆库鲁克塔格地区辛格尔变质核杂岩特征

辛格尔一带的前震旦纪古老变质岩系为一系列的变质核杂岩。其内核由早—中太古代古陆核组成 ,外核为晚太古代、古元古代、中元古代、新元古代早期变质岩系构成的多层次构造增生楔 ,并有多期剥离断层配套。新元古宙代晚期震旦系为盖层。它是地壳多期伸缩构造的产物。变质核杂岩体是一个有利于成矿的构造体系。辛格尔变质核杂岩构造是一个对有色和贵金属等矿产成矿的有利构造     

Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: An illite crystallinity and Mite bo, lattice spacing study

Abstract Illite crystallinity (IC) and illite b, lattice spacing were measured across the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan. For the IC survey, 103 samples of argillaceous rocks were analyzed from the central area and the western area of the belt. Values of IC (Kubler Index) vary between 0.28 and 0.71 Δ°2θ and indicate diagenetic and anchizone metamorphism respectively. The IC distribution reveals two contrasting patterns of thermal maturity. The Hanazono Formation, exposed in the northern area of the belt, generally dips north, but IC values increase systematically from 0.28 Δ°2θ in the north to 0.54 Δ°2θ in the south and indicate an inverted thermal structure. Values in other formations vary widely in the southern area of the belt ranging between 0.45 and 0.71 Δ°2θ, but the values do not show any systematic change from north to south and on average remain almost constant. Illite b_o, lattice spacing values for 56 samples vary between 9.006 and 9.041 Å corresponding to intermediate pressure conditions of the metamorphic facies. These values, combined with paleotemperatures estimated from IC, indicate paleogeothermal gradients of 22 and 31°C/km for the northern and southern areas of the belt, respectively. The inverted thermal structure of the Hanazono Formation, together with a lower paleogeothermal gradient, possibly is a result of the subduction of a relatively cold oceanic plate during the Late Cretaceous. The higher geothermal gradient could be a product of late thermal overprinting caused by the later subduction of a comparatively younger and hotter oceanic plate during the Eocene.     

Pacific-type orogeny revisited: Miyashiro-type orogeny proposed

Abstract The concept of Pacific-type orogeny is revised, based on an assessment of geologic data collected from the Japanese Islands during the past 25 years. The formation of a passive continental margin after the birth of the Pacific Ocean at 600 Ma was followed by the initiation of oceanic plate subduction at 450 Ma. Since then, four episodes of Pacific-type orogeny have occurred to create an orogenic belt 400 km wide that gradually grew both oceanward and downward. The orogenic belt consists mainly of an accretionary complex tectonically interlayered with thin (<2 km thick), subhorizontal, high-P/T regional metamorphic belts. Both the accretionary complex and the high-P/T rocks were intruded by granitoids ～100 million years after the formation of the accretionary complex. The intrusion of calc-alkaline (CA) plutons was synchronous with the exhumation of high-P/T schist belts. Ages from microfossils and K-Ar analysis suggest that the orogenic climax happened at a time of mid-oceanic ridge subduction. The orogenic climax was characterized by the formation of major subhorizontal orogenic structures, the exhumation of high-P/T schist belts by wedge extrusion and subsequent domed uplift, and the intrusion-extrusion of CA magma dominantly produced by slab melting. The orogenic climax ended soon after ridge subduction, and thereafter a new Pacific-type orogeny began. A single Pacific-type orogenic cycle may correspond to the interaction of the Asian continental margin with one major Pacific oceanic plate. Ophiolites in Japan occur as accreted material and are not of island-arc but of plume origin. They presumably formed after the birth of the southern Pacific superplume at 600 Ma, and did not modify the cordilleran-type orogeny in a major way. Microplates, fore-arc slivers, intra-oceanic arc collisions and the opening of back-arc basins clearly contributed to cordilleran orogenesis. However, they were of secondary importance and served only to modify pre-existing major orogenic components. The most important cause of cordilleran-type orogeny is the subduction of a mid-oceanic ridge, by which the volume of continental crust increases through the transfer of granitic melt from the subducting oceanic crust to an orogenic welt. Accretionary complexes are composed mainly of recycled granitic sediments with minor amounts of oceanic material, which indicate that the accretion of oceanic material, including huge oceanic plateaus, was not significant for orogenic growth. Instead, the formation and intrusion of granitoids are the keys to continental growth, which is the most important process in Pacific-type orogeny. Collision-type orogeny does not increase the volume of continental crust. The name ‘Miyashiro-type orogeny’ is proposed for this revised concept of Pacific-type or cordilleran-type orogeny, in order to commemorate Professor A. Miyashiro's many contributions to a better understanding of orogenesis.     

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.     

长度计量仪器测距指标处理系统的开发运用

测绘行业近几年发生了巨大的变化 ,传统的测量方法大多被数字化手段所代替 ,然而部分测绘仪检部门在处理测距仪器检定数据时有的采用自行编制的功能不甚完备的程序进行计算。针对这种情况 ,作者开发了长度计量仪器测距指标处理系统 (KRA) ,该系统能较为全面地满足光电测量仪器检测数据的后期处理。本文就该系统的开发和运用作了详细阐述。