山西地区未来3年地震强度预测

选取１９７０年至１９９７年时间段内发生在山西地区的逐年最大震级,采用门限自回归原理进行建模,经过反复对比,选取最佳模型,预测得知该区未来３年（至２０００年）发生强震（Ｍｓ＞６．０）的可能性不大。     

起算震级与衰减不确定性校正

分析表明,起算震级Ｍ０对危险性分析结果的表现为当Ｍ０取取值不够低时,场地的危险性将被低估。此种低估影响与场地的地震危险性高低潜源距场地的距离及衰减公式的形式和标准差有关。     

安徽巢县南陵湖组火山碎屑流沉积物的发现

在安徽巢县下三叠统南陵湖组上部，发育着一套火山碎屑沉积物，主要由英安质角砾岩、英安质晶屑、玻屑凝灰角砾岩和英安质玻屑、晶屑凝灰岩等组成。它厚１．８ｍ，层序清楚，分为２个布玛旋回。研究表明，它属于海底喷发、水流搬运的火山碎清流沉积（火山浊流沉积），并且可能是印支早期华北板块与扬子板块相互碰撞的一种反映。     

The concept of dominant discharge applied to two gravel-bed streams in relation to channel stability thresholds

Dominant discharge may be defined as that discharge which transports most bed sediment in a stream that is close to steady-state conditions. The concept is examined in relation to two single thread gravel-bedded streams. One stream is alluvial and free to adjust its geometry whilst in the other, channel capacity and form are partially constrained by cohesive till-banks and a heavily compacted bed. The total quantity of bedload and its calibre were measured for every flood over a six year period, so that the relationship between the grain-size of bedload and the most effective discharge could be examined in the context of thresholds for channel change. The dominant discharge concept was applicable to the alluvial stream in that the bankfull value is an effective discharge for maintaining channel capacity. The concept applied less well to the ‘non-alluvial’ stream. Although in both streams the bankfull value was exceeded for less than 0.34 per cent of the time, overbank flows are important in instigating channel change. It is only during overbank flows that the largest bed material is entrained in quantity. For within-channel flows a threshold separates flows which winnow fine matrix from those which entrain the finer bed gravels. This threshold occurred at 60 per cent bankfull. It was concluded that the dominant discharge concept can be applied to streams close to steady-state which are alluvial, competent, and free to adjust their boundaries. An important proviso is that two channel-stability domains can be recognized. These domains represent channel maintenance and channel adjustment and are defined by intrinsic thresholds in the bed material entrainment function.     

Threshold relations for the transport of sediment by overland flow on desert hillslopes

Although the Shields relation was developed for rivers, it has been applied to sediment transport by overland flow. According to the Shields relation, where the critical boundary Reynolds number Re_*c exceeds 40, the critical Shields number F_*c is independent of both Re_*c and the ratio of the critical flow depth to particle diameter d_c/D. Analyses of data collected from runoff plots in southern Arizona reveal that F_*c is positively correlated with both Re_*c and d_c/D. Thus the Shields relation does not apply to overland flow on debris-covered desert hillslopes. Multiple regression analysis is employed to develop alternative threshold relations in which critical boundary shear stress τ_c is related to D and d_c/D (R² = 0.782) and to D and S_c (critical gradient) (R² = 0.625). The computed R² values derive in large part from the spurious correlations of d_c/D and S_c with τ_c. Nevertheless, the relations may be utilized to predict τ_c. In this regard, the latter relation is likely to prove more useful than the former because S_c is generally known, whereas d_c is not. An investigation of the functional relation between τ_c and D reveals that τ_c varies approximately with D² for overland flow on the desert hillslopes under study, whereas the Shields relation predicts a linear relation (i.e. a D exponent of 1). This result is consistent with Cheng's data which show that F_*c varies with (d_c/D)^?1 where 0.4 < d_c/D < 2 and may be explained in terms of increased energy dissipation both in separation zones downslope of particles and in distortion of the water surface as d_c/D decreases. Consequently, larger values of τ_c, and hence F_*c, are required to initiate the transport of particles of a given size D as d_c decreases.     

Sedimentologic and stratigraphic constraints on emplacement of the Star Kimberlite, east–central Saskatchewan

Diamond-bearing kimberlites in the Fort à la Corne region, east–central Saskatchewan, consist primarily of extra-crater pyroclastic deposits which are interstratified with Lower Cretaceous (Albian and Cenomanian) marine, marginal marine and continental sediments. Approximately 70 individual kimberlite occurrences have been documented. The Star Kimberlite, occurring at the southeastern end of the main Fort à la Corne trend, has been identified as being of economic interest, and is characterized by an excellent drill core database. Integration of multi-disciplinary data-sets has helped to refine and resolve models for emplacement of the Star Kimberlite. Detailed core logging has provided the foundation for sedimentological and volcanological studies and for construction of a regionally consistent stratigraphic and architectural framework for the kimberlite complex. Micropaleontologic and biostratigraphic analysis of selected sedimentary rocks, and U–Pb perovskite geochronology on kimberlite samples have been integrated to define periods of kimberlite emplacement. Radiometric age determination and micropaleontologic evidence support the hypothesis that multiple kimberlite eruptive phases occurred at Star. The oldest kimberlite in the Star body erupted during deposition of the predominantly continental strata of the lower Mannville Group (Cantuar Formation). Kimberlites within the Cantuar Formation include terrestrial airfall deposits as well as fluvially transported kimberlitic sandstone and conglomerate. Successive eruptive events occurred contemporaneous with deposition of the marginal marine upper Mannville Group (Pense Formation). Kimberlites within the Pense Formation consist primarily of terrestrial airfall deposits. Fine- to medium-grained cross-stratified kimberlitic (olivine-dominated) sandstone in this interval reflects reworking of airfall deposits during a regional marine transgression. The location of the source feeder vents of the Cantuar and Pense kimberlite deposits has not been identified. The youngest and volumetrically most significant eruptive events associated with the Star Kimberlite occur within the predominantly marine Lower Colorado Group (Joli Fou and Viking Formations). Kimberlite beds, which occur at several horizons within these units, consist of subaerial and marine fall deposits, the latter commonly exhibiting evidence of wave-reworking. Black shale-encased resedimented kimberlite beds, likely deposited as subaqueous debris flows and turbidites, are particularly common in the Lower Colorado Group. During its multi-eruptive history, the Star Kimberlite body is interpreted to have evolved from a feeder vent and overlying positive-relief tephra ring, into a tephra cone. Initial early Joli Fou volcanism resulted in formation of a feeder vent (200 m diameter) and tephra ring. Subsequent eruptions, dominated by subaerial deposits, partly infilled the crater and constructed a tephra cone. A late Joli Fou eruption formed a small (70 m diameter) feeder pipe slightly offset to the NW of the early Joli Fou feeder vent. Deposits from this event further infilled the crater, and were deposited on top of early Joli Fou kimberlite (proximal to the vent) and sediments of the Joli Fou Formation (distal to the vent). The shape of the tephra cone was modified during multiple marine transgression and regression cycles coeval with deposition of the Lower Colorado Group, resulting in wave-reworked kimberlite sand along the fringes of the cone and kimberlitic event deposits (tempestites, turbidites, debris flows) in more distal settings.     

The Early Palaeozoic Break-up of Northern Gondwana: Sedimentology, Physical Volcanology and Geochemistry of a Submarine Volcanic Complex in the Bavarian Facies Association, Saxothuringian Basin, Germany

Ordovician volcano-sedimentary successions of the Bavarian facies association in the Saxothuringian basin record the continental rift phase of the separation of the Saxothuringian Terrane from Gondwana. An 80 m succession from the Vogtendorf beds and Randschiefer Series (Arenig-Middle Ordovician), exposed along the northern margin of the Münchberg Gneiss Massif in northeast Bavaria, were subjected to a study of their sedimentology, physical volcanology and geochemistry. The Randschiefer series previously has been interpreted as lavas, tuffs, sandstones and turbidites, but the studied Ordovician units include four main lithological associations: mature sandstones and slates, pillowed alkali-basalts and derivative mass flow deposits, trachyandesitic lavas and submarine pyroclastic flow deposits interbedded with turbidites. Eight lithofacies have been distinguished based on relict sedimentary structures and textures, which indicate deposition on a continental shelf below wave base. The explosive phase that generated the pyroclastic succession was associated with the intrusion of dykes and sills, and was succeeded by the eruption of pillowed basalts. Debris flow deposits overlie the basalts. Ordovician volcanism in this region, therefore, alternated between effusive and explosive phases of submarine intermediate to mafic volcanism.

Based on geochemical data, the volcanic and pyroclastic rocks are classified as basalts and trachyandesites. According to their geochemical characteristics, especially to their variable concentrations of incompatible elements such as the High Field Strength Elements (HFSE), they can be divided into three groups. Group I, which is formed by massive lavas at the base of the succession, has extraordinarily high contents of HFSE. The magmas of this group were probably derived from a mantle source in the garnet stability field by low (ca. 1%) degrees of partial melting and subsequent fractionation. Group II, which comprises the pillow lavas at the top of the sequence, displays moderate enrichment of HFSE. This can be explained by a slightly higher degree of melting (ca. 1.6%) for the primary magma. Group I and II melts fractionated from their parental magmas in different magma chambers. The eruption centres of Groups I and II, therefore, cannot be the same, and the volcanic rocks must have originated from different vents. The sills and pyroclastic flow deposits of Group III stem at least partly from the same source as Group I. Rocks of Group I most likely mixed together with Group II components during the formation of the Group III flows, which became hybridised during eruption, transportation and emplacement.

The sedimentological and geochemical data best support a rift as the tectonic setting of this volcanism, analogous to modern continental rift zones. Hence, the rift-associated volcanic activity preserved in the Vogtendorf beds and Randschiefer Series represents an early Ordovician stage of rift volcanism when the separation of the Saxothuringian Terrane from Gondwana had just commenced.     

Timing of magma extraction during the Campanian Ignimbrite eruption (Campi Flegrei Caldera)

A core drilled within the northern part of the city of Napoli has offered the unique opportunity to observe in one single sequence the superposition of the four pyroclastic flow units emplaced during the Campanian Ignimbrite (CI) eruption. Such a stratigraphic succession has never been encountered before in natural or in man made exposures. Therefore the CI sequence was reconstructed only on the basis of stratigraphic correlations and compositional data (in literature). The occurrence of four superposed CI flows, together with all the data available (in literature) allowed us to better constrain the chemical stratigraphy of the deposit and the compositional structure of the CI magma chamber. The CI magma chamber includes two cogenetic magma layers, separated by a compositional gap. The upper magma layer was contaminated by interaction with radiogenic fluids. The two magma layers were extruded either individually or simultaneously during the course of the eruption. In the latter case they produced a hybrid magma. But no evidence of input of new geochemically and isotopically distinct magma batches just prior or during the eruption has been found. Comparison with the exposed CI deposits has permitted reconstruction of variable eruption phases and related magma withdrawal and caldera collapse episodes. The eruption was likely to have began with phreatomagmatic explosions followed by the formation of a sustained plinian eruption column fed by the simultaneous extraction from both magma layers. Towards the end of this phase the upward migration of the fragmentation surface and the decrease in magma eruption rate and/or activation of fractures formed an unstable pulsating column that was fed only by the most-evolved magma layer. This plinian phase was followed by the collapse of the eruption column and the beginning of caldera formation. At this stage expanded pyroclastic flows fed by the upper magma layer in the chamber generated. During the following major caldera collapse episode, the maximum mass discharge rate was reached and both magma layers were tapped, generating expanded pyroclastic flows. Towards the end of the eruption, only the deeper and less differentiated magma layer was tapped producing more concentrated pyroclastic flows that traveled short distances.     

长白山天池火山一次近代喷发物的特征

长白山天池火山是中国最具有潜在灾害性喷发危险的活动火山。在开展长白山天池火山近代喷发历史的研究中,通过野外考察、粒度分析、岩石化学研究,识别出了一套新的火山喷发物。这套喷发物分布于天池水面东北侧,为一套灰色多层火山碎屑堆积,厚约9.2m。下伏公元1668年的火山空降堆积。粒度分析表明,天池火山最近一次喷发物以空降堆积为主,夹一层薄层涌浪堆积,火山喷发类型为射气岩浆型。涌浪堆积碎屑物的分数维值为2.71。空降堆积的分数维值小于涌浪堆积,综合投点求出的分数维值为2.36。显微镜下可观察到鸡骨架状玻屑,无黏土矿物,为原生火山爆发堆积。火山碎屑堆积物中的浮岩岩石化学分析结果表明岩浆成分为粗面质。根据历史记录、地层层序关系、堆积物特征的综合分析,推测堆积物的形成时间为公元1903年     

Geochemical Characteristics of Early Permian Pyroclastic Rocks in the Jimunai Basin,West Junggar,Xinjiang(NW China): Implications for Provenance and Tectonic Setting

During the Late Paleozoic, West Junggar(Xinjiang, NW China) experienced a shift in tectonic setting from compression to extension. Ha'erjiao is an important area for investigating collisional structures, post–collisional structures, and magmatic activities. Based on the petrological and geochemical characteristics of pyroclastic and other volcanic rocks in the Permian Kalagang Formation from the borehole ZKH1205 in the Jimunai Basin, the main types of source rock for the pyroclastic rocks deposited in the basin are identified and their implications for the Early Permian tectonic setting examined. The abundance of basalt and andesite lithic fragments in the pyroclastic rocks, together with the REE characteristics and the contents of transition and high field strength elements show that the source rocks were chiefly intermediate–basic volcanic rocks. High ICV values, low CIA values, low Rb/Sr ratios, low Th/U ratios and the mineralogical features suggest weak chemical weathering of the source rocks; the geochemical patterns of the pyroclastic rocks might not only have been impacted by crustal contamination but also might be related to the nature of the magma from the source area. The geochemical properties of the pyroclastic rocks distinguish them from arc-related ones, and such samples plot in the within-plate basalt(WPB) field in some diagrams. This is consistent with the formation background of the Early Permian volcanic rocks in this region.