Volcanism and sedimentation in part of a Late Archaean rift: the Hartbeesfontein basin, Transvaal, South Africa

The Hartbeesfontein basin is one basin within the Late Archaean rift system of South Africa. This rift system has been recently compared to the Basin and Range province in western North America and may therefore be an ensialic extensional back-arc basin. Structurally, the Hartbeesfontein basin is a half-graben structure bounded to the south-east by a major, normal, listric fault and to the north-east and south-west by strike-slip (transfer?) fault zones. It is infilled by over 2000 m of diamictites, shales, lavas and chemical sediments. Initial basin formation appears to be accompanied by phreatomagmatic volcanic activity caused by the interaction between basic tholeiitic magmas rising along fractures and groundwater. Volcaniclastic debris from these eruptions was incorporated into laharic debris flows and deposited on basin marginal alluvial fans. At the same time a deep, permanent lake formed within the basin in which silts and muds accumulated. Major fissure eruptions of basic, tholeiitic lavas followed, their eruptive centres being apparently located along the strike-slip (transfer?) fault /ones. Initially, these fissure eruptions had high rates of magma discharge accompanied by intense fire fountaining that resulted in the rapid accumulation of aa type flows. Later lava discharge rates decreased and more quiescent pahoehoe type flows were erupted. Localized centres of acid volcanism within the basic lava pile were located along the south-western strike-slip fault zone. These acid volcanic rocks are interpreted as co-ignimbrite lag breccias and pyroclastic flow deposits and tuffs produced by the repeated formation and collapse of Plinian eruption columns. Towards the top of the basic lava pile, two breaks in volcanism permitted the formation of dolomitic playa lakes. Sedimentation in these lakes was terminated by further basic lava flows. At the top of the basin fill sequence is a thick, bedded chert interpreted as a magadiitic, alkaline playa lake fed by silica-rich hot springs located along the south-eastern edge of the basin. Quartzites and conglomerates deposited by braided rivers unconformably overlie the basin-fill sequence and probably represent a through flowing river system signifying termination of the Hartbeesfontein basin as a separate basin. The Hartbeesfontein basin and its fill demonstrate that a close relationship exists between fissure volcanism, sedimentation and basin evolution and that the strike-slip, transfer faults acted as the loci of volcanic activity.     

The Miocene Saint-Florent Basin in northern Corsica: stratigraphy, sedimentology, and tectonic implications

Late early–early middle Miocene (Burdigalian–Langhian) time on the island of Corsica (western Mediterranean) was characterized by a combination of (i) postcollisional structural inversion of the main boundary thrust system between the Alpine orogenic wedge and the foreland, (ii) eustatic sealevel rise and (iii) subsidence related to the development of the Ligurian‐Provençal basin. These processes created the accommodation for a distinctive continental to shallow‐marine sedimentary succession along narrow and elongated basins. Much of these deposits have been eroded and presently only a few scattered outcrop areas remain, most notably at Saint‐Florent and Francardo. The Burdigalian–Langhian sedimentary succession at Saint‐Florent is composed of three distinguishing detrital components: (i) siliciclastic detritus derived from erosion of the nearby Alpine orogenic wedge, (ii) carbonate intrabasinal detritus (bioclasts of shallow‐marine and pelagic organisms), and (iii) siliciclastic detritus derived from Hercynian‐age foreland terraines. The basal deposits (Fium Albino Formation) are fluvial and composed of Alpine‐derived detritus, with subordinate foreland‐derived volcanic detritus. All three detrital components are present in the middle portion of the succession (Torra and Monte Sant'Angelo Formations), which is characterized by thin transitional deposits evolving vertically into fully marine deposits, although the carbonate intrabasinal component is predominant. The Monte Sant'Angelo Formation is characteristically dominated by the deposits of large gravel and sandwaves, possibly the result of current amplification in narrow seaways that developed between the foreland and the tectonically collapsing Alpine orogenic wedge. The laterally equivalent Saint‐Florent conglomerate is composed of clasts derived from the late Permian Cinto volcanic district within the foreland. The uppermost unit (Farinole Formation) is dominated by bioclasts of pelagic organisms. The Saint‐Florent succession was deposited during the last phase of the counterclockwise rotation of the Corsica–Sardinia–Calabria continental block and the resulting development of the Provençal oceanic basin. The succession sits at the paleogeographic boundary between the Alpine orogenic wedge (to the east), its foreland (to the west), and the Ligurian‐Provençal basin (to the northwest). Abrupt compositional changes in the succession resulted from the complex, varying interplay of post‐collisional extensional tectonism, eustacy and competing drainage systems.     

Application of seismic and sequence stratigraphic concepts to a lava‐fed delta system in the Faroe‐Shetland Basin,UK and Faroes

Detailed seismic stratigraphic analysis of 2D seismic data over the Faroe‐Shetland Escarpment has identified 13 seismic reflection units that record lava‐fed delta deposition during discrete periods of volcanism. Deposition was dominated by progradation, during which the time shoreline migrated a maximum distance of ～44 km in an ESE direction. Localised collapse of the delta front followed the end of progradation, as a decrease in volcanic activity left the delta unstable. Comparison with modern lava‐fed delta systems on Hawaii suggests that syn‐volcanic subsidence is a potential mechanism for apparent relative sea level rise and creation of new accommodation space during lava‐fed delta deposition. After the main phase of progradation, retrogradation of the delta occurred during a basinwide syn‐volcanic relative sea level rise where the shoreline migrated a maximum distance of ～75 km in a NNW direction. This rise in relative sea level was of the order of 175–200 m, and was followed by the progradation of smaller, perched lava‐fed deltas into the newly created accommodation space. Active delta deposition and the emplacement of lava flows feeding the delta front lasted ～2600 years, although the total duration of the lava‐fed delta system, including pauses between eruptions, may have been much longer.     

Geomagnetic secular variation in historical lavas from the Canary Islands

Summary. The age of historical lava flows on the Canary Islands can be traced back at least to 1585 ad . During the last 400 years eruptions occurred at fairly regular intervals. The palaeomagnetic directions of these lavas record the ambient field direction with high precision so that an historical secular variation curve (SVC) can be constructed for the Canaries which closely resembles the curve registered at Paris. The SVC can be used as a dating tool for historical eruptions of unknown age and can probably be extended into the last 4000–5000 yr of recent volcanic activity on the Canaries.     

Lyttelton Volcano, Banks Peninsula, New Zealand: Primary volcanic landforms and eruptive centre identification

Lyttelton Volcano, Banks Peninsula, New Zealand, has historically been viewed as a simple volcanic cone. This paper uses digital terrain models (DTM) and primary volcanic landforms to reinterpret Lyttelton Volcano as having multiple eruptive centres. Primary volcanic landforms are features produced during active volcanism, classified as constructional, hypabyssal, and erosional volcanic features. Constructional volcanic features are lava flows, scoria cones and domes; hypabyssal volcanic features are dykes and sills; and erosional volcanic features are valley and ridge patterns and orientations. Lava flow trends are recognised from aerial photograph analysis and supported by field observations, highlighting radiating lava trends around specific locations within Lyttelton Harbour. Scoria cones and domes occur on the outer flanks of volcanic cones, and are used as such in the identification of remnant cone surfaces. Dyke orientations are plotted and then projected to the interior of the volcano, defining 13 zones of convergence. The projected arrays of these orientations highlight defined regions along the erosional crater rim, each indicating a radial dyke swarm, from which the projected trends of the associated dykes indicate an eruptive centre. Valley and ridge orientations are projected from the longest valley or ridge segment, towards the inner harbour. Radiating erosive patterns are incepted during the growth and degradation of a volcanic cone, with the resulting trends orienting to the summit. Zones of convergence/eruptive centres are identified from lava flow orientations, onlapping lava sequences, scoria cones, and intrusive locations. The summit of a volcanic edifice can be identified from the orientations of valleys and ridges, while radial dyke systems determine whether this summit was a volcanic centre or simply a local topographic high. Volcanic landforms are used to identify cone sectors, the preserved sector associated with a particular eruptive centre. Cone sector limits are defined by a basal footprint and an erosional crater rim, with similar arcuate features (remnant cone features) being exposed in the interior of the volcano. Lyttelton Volcano comprises fifteen volcanic cones, with vent locations controlled by underlying fault lineaments. The growth and erosion of each volcanic cone is reflected in primary volcanic landforms, with the preserved features of cones confined to cone sectors and cone artefacts.     

山东临朐、昌乐地区晚第三纪火山地貌

山东临朐、昌乐地区是渤海盆地南缘,郯庐断裂带中段西侧晚第三纪火山地貌集中分布的地区之一。本文对区内火山地貌类型和地貌发育过程作了重点探讨。     

山东临朐、昌乐地区晚第三纪火山地貌

山东临朐、昌乐地区是渤海盆地南缘,郯庐断裂带中段西侧晚第三纪火山地貌集中分布的地区之一。本文对区内火山地貌类型和地貌发育过程作了重点探讨。     

Impact of a pre-existing transverse drainage system on active rift stratigraphy: An example from the Corinth Rift,Greece

Models to explain alluvial system development in rift settings commonly depict fans that are sourced directly from catchments formed in newly uplifted footwalls, which leads to the development of steep-sided talus-cone fans in the actively subsiding basin depocentre. The impact of basin evolution on antecedent drainage networks orientated close to perpendicular to a rift axis, and flowing over the developing hangingwall dip slope, remains relatively poorly understood. The aim of this study is to better understand the responses to rift margin uplift and subsequent intrabasinal fault development in determining sedimentation patterns in alluvial deposits of a major antecedent drainage system. Field-acquired data from a coarse-grained alluvial syn-rift succession in the western Gulf of Corinth, Greece (sedimentological logging and mapping) has allowed analysis of the spatial distribution of facies associations, stratigraphic architectural elements and patterns of palaeoflow. During the earliest rifting phase, newly uplifted footwalls redirected a previously established fluvial system with predominantly southward drainage. Footwall uplift on the southern basin margin at an initially relatively slow rate led to the development of an overfilled basin, within which an alluvial fan prograded to the south-west, south and south-east over a hangingwall dip slope. Deposition of the alluvial system sourced from the north coincided with the establishment of small-scale alluvial fans sourced from the newly uplifted footwall in the south. Deposits of non-cohesive debris flows close to the proposed hangingwall fan apex pass gradationally downstream into predominantly bedload conglomerate deposits indicative of sedimentation via hyperconcentrated flows laden with sand- and silt-grade sediment. Subsequent normal faulting in the hangingwall resulted in the establishment of further barriers to stream drainage, blocking flow routes to the south. This culminated in the termination of sediment supply to the basin depocentre from the north, and the onset of underfilled basin conditions as signified by an associated lacustrine transgression. The evolution of the fluvial system described in this study records transitions between three possible end-member types of interaction between active rifting and antecedent drainage systems: (a) erosion through an uplifted footwall, (b) drainage diversion away from an uplifted footwall and (c) deposition over the hangingwall dip slope. The orientation of antecedent drainage pathways at a high angle to the trend of a developing rift axis, replete with intrabasinal faulting, exerts a primary control on the timing and location of development of overfilled and underfilled basin states in evolving depocentres.     

Sequence of faulting and deformation during the 2000 eruptions of the Usu Volcano, Hokkaido, Japan—interpretation and image analyses of aerial photographs

Significant faulting and deformation of the ground surface has been rarely known during volcanic eruptions. Usu Volcano, Hokkaido, Japan, is a unique example of deformation due to felsic magma intrusion. Usu Volcano has a history of such types of eruptions as phreatic, pumice eruption (Plinian type), pyroclastic flowing and lava doming since 1663. On March 31, 2000, phreatomagmatic to phreatic eruptions took place after 23 years of dormancy in the western piedmont, followed by explosions on the western flank of Usu Volcano. They were associated with significant deformation including faulting and uplift. The eruptions and deformation were continuing up to the end of May 2000.We identified the faulting using total nine sets of aerial photographs taken from before the eruption (March 31, 2000) to more than 1 year (April 27, 2001) after the end of the activity, and traced deformation processes through image processing using aerial photographs. We found that some of the new faults and the associated phreatic eruptions were related to old faults formed during the 1977–1981 eruptive episode.The image processing has revealed that the surface deformation is coincident with the area of faulting forming small grabens and the phreatic explosion vents. However, the faulting and main explosive eruptions did not take place in the highest uplift area, but along the margin. This suggests that the faulting and explosive activities were affected by small feeder channels diverging from the main magma body which caused the highest uplift.     

Integrated provenance record of a forearc basin modified by slab‐window magmatism: detrital‐ zircon geochronology and sandstone compositions of the Paleogene Arkose Ridge Formation,south‐central Alaska

This study presents an integrated provenance record for ancient forearc strata in southern Alaska. Paleocene–Eocene sedimentary and volcanic strata >2000 m thick in the southern Talkeetna Mountains record nonmarine sediment accumulation in a remnant forearc basin. In these strata, igneous detritus dominates conglomerate and sandstone detrital modes, including plutonic and volcanic clasts, plagioclase feldspar, and monocrystalline quartz. Volcanic detritus is more abundant and increases upsection in eastern sandstone and conglomerate. U‐Pb ages of >1600 detrital zircons from 19 sandstone samples document three main populations: 60–48 Ma (late Paleocene–Eocene; 14% of all grains), 85–60 Ma (late Cretaceous–early Paleocene; 64%) and 200–100 Ma (Jurassic–Early Cretaceous; 11%). Eastern sections exhibit the broadest distribution of detrital ages, including a principal population of late Paleocene–Eocene ages. In contrast, central and western sections yield mainly late Cretaceous–early Paleocene detrital ages. Collectively, our results permit reconstruction of individual fluvial drainages oriented transverse to a dissected arc. Specifically, new data suggest: (1) Detritus was eroded from volcanic‐plutonic sources exposed along the arcward margin of the sampled forearc basin fill, primarily Jurassic–Paleocene magmatic‐arc plutons and spatially limited late Paleocene–Eocene volcanic centers; (2) Eastern deposystems received higher proportions of juvenile volcanic detritus through time from late Paleocene–Eocene volcanic centers, consistent with emplacement of a slab window beneath the northeastern part of the basin during spreading‐ridge subduction; (3) Western deposystems transported volcanic‐plutonic detritus from Jurassic–Paleocene remnant arc plutons and local eruptive centers that flanked the northwestern part of the basin; (4) Diagnostic evidence of sediment derivation from accretionary‐prism strata exposed trenchward of the basin fill is lacking. Our results provide geologic evidence for latest Cretaceous–early Paleocene exhumation of arc plutons and marine forearc strata followed by nonmarine sediment accumulation and slab‐window magmatism. This inferred history supports models that invoke spreading‐ridge subduction beneath southern Alaska during Paleogene time, providing a framework for understanding a mature continental‐arc/forearc‐basin system modified by ridge subduction. Conventional provenance models predict reduced input of volcanic detritus to forearc basins during progressive exhumation of the volcanic edifice and increasing exposure of subvolcanic plutons. In contrast, our results show that forearc basins influenced by ridge subduction may record localized increases in juvenile volcanic detritus during late‐stage evolution in response to accumulation of volcanic sequences formed from slab‐window eruptive centers.     

西南极利文斯顿岛汉那角火山岩地质及火山活动

本文以野外地质资料为基础，并结合岩相学特征，论述了西南极南设得兰群岛利文斯顿岛汉那角（ＨａｎｎａｈＰｏｉｎｔ）火山岩地层及岩石的基本特征，初步讨论了该地火山活动过程。该地互层状产出的火山角砾岩和熔岩，为南极半岛火山岩组的一部分。剖面上出露的三个火山岩地层旋回分别是三期火山作用的产物。第一、二旋回岩层产状的变化记录了火山作用过程中的一次活动间歇。第一旋回的英安岩和含有流纹岩质熔岩夹层的凝灰角砾岩仅为第一期火山活动末的产物；第二旋回为三次较强喷发活动的产物，从安山岩质火山碎屑岩为主，经玄武岩－安山岩质熔岩为主，过渡为含有安山岩或流纹岩质熔浆团的火山角砾岩。第三期火山活动可能更强烈，大面积分布的厚层玄武质熔岩无论从产状上还是在岩石学特征方面都有别于其他旋回。火山岩地层总体产状稳定，平缓地倾向北西，表明该区未受到过明显的构造影响，火山活动中心可能有逐步向北西迁移的趋势，与南设得兰群岛岩浆弧演变趋势一致。次火山岩主要为玄武岩质小岩墙，集中在第二旋回底部。第一、二旋回接触界线两侧凝灰角砾岩表现较强的沸石化。这些现象说明该接触面附近为一薄弱带，侵入活动和蚀变均较易发生。     

Impact of volcanism on the sedimentary record of the Neuquén rift basin,Argentina: towards a cause and effect model

The analysis of volcano‐sedimentary infill in sedimentary basins constitutes a challenge for basin analysis and hydrocarbon exploration worldwide. In order to understand the contribution of volcanism to the sedimentary record in rift basins, we study the Jurassic effusive‐explosive volcanic infill of an inverted extensional depocentre at the Neuquén Basin, Argentina. A cause and effect model that evaluates the relationship between volcanism and sedimentation was devised to develop a conceptual model for the tectono‐stratigraphic evolution of this volcanic rift basin. We show how the variations in the volcanism, coupled with the activity of extensional faults, determined the types of volcanic edifices (i.e., composite volcanoes, graben‐calderas, and lava fields). Volcanic edifices controlled the stacking patterns of the volcanic units as well as sedimentary systems. The landform of the volcanic edifices, as well as the styles and scales of the eruptions governed the sedimentary input to the basin, setting the main variables of the sedimentary systems, such as provenance, grain size, transport and deposition and geometry. As a result, the contrasting volcaniclastic input, from higher volcaniclastic input to lower volcaniclastic input, associated with different subsidence patterns, determined the high‐resolution syn‐rift infill patterns of the extensional depocentre. The cause and effect model presented in this study isolates the variables of the volcanic environments that control the sedimentary scenarios. We suggest that, by adjusting the first order input parameters of the model, these cause and effect scenarios could be adapted to similar rift basins, in order to establish predictive facies models with stratigraphic controls, and the impact of volcanism on their stratigraphic records.     

Age,stratigraphy, sedimentology and tectonic setting of the Sigri Pyroclastic Formation and its fossil forests,Early Miocene,Lesbos, Greece

The Petrified Forest of Lesbos comprises silicified tree fossils at multiple stratigraphic levels within the Lower Miocene Sigri Pyroclastic Formation. Our objective was to understand the interplay of tectonic setting, structural evolution, volcanological setting and basin evolution in the preservation of this remarkable natural monument. Sections were logged for lithology, sedimentary structures and hydrothermal alteration. Orientations of fallen fossil trees were measured. Samples were taken for mineralogical and geochemical analysis. ⁴⁰Ar/³⁹Ar dating was carried out on mineral separates from four samples. Widespread andesite‐dacite domes, the Eressos Formation, intrude and overlie metamorphic basement and are overlain by the Sigri Pyroclastic Formation, which comprises several hundreds of metres of pyroclastic flow tuffs (unwelded ignimbrites) interbedded with fluvial conglomerate and volcaniclastic sandstone. The Sigri Pyroclastic Formation ranges in age from 21.5 to 22 Ma, where it overlies the lacustrine Gavathas Formation, to younger than 18.4 Ma. Tuffs and fluvial conglomerates in the Sigri Pyroclastic Formation coarsen eastwards, and petrified trees and soil horizons occur throughout the Formation. The recurrence of pyroclastic flows was approximately one every 20 ka, so destructive flows were relatively infrequent, allowing the development of climax vegetation between most eruptions. Conglomerate‐filled channels show that rivers flowed westwards. Tree fall directions indicate NW to N movement of pyroclastic flows, implying a source near the younger Mesotopos–Tavari caldera to the south. The basin, which formed in a NNE‐trending dextral strike‐slip regime, provided some topographic steering. Following the Sigri Pyroclastic Formation at ca. 18 Ma, there was a rapid increase in the pace of volcanic activity, with the eruption of thick lava sequences and welded ignimbrites, and intrusion of dykes and laccoliths in SW Lesbos. Rapid burial by permeable tuffs, silica from alteration of volcanic ash, and later hydrothermal circulation all contributed to the preservation of the petrified trees.     

Variation in style of magmatism and emplacement mechanism induced by changes in basin environments and stress fields (Pannonian Basin,Central Europe)

The development of high‐resolution 3D seismic cubes has permitted recognition of variable subvolcanic features mostly located in passive continental margins. Our study area is situated in a different tectonic setting, in the extensional Pannonian Basin system (central Europe) where the lithospheric extension was associated with a wide variety of magmatic suites during the Miocene. Our primary objective is to map the buried magmatic bodies, to better understand the temporal and spatial variation in the style of magmatism and emplacement mechanism within the first order Mid‐Hungarian Fault Zone (MHFZ) along which the substantial Miocene displacement took place. The combination of seismic, borehole and log data interpretation enabled us to delineate various previously unknown subvolcanic‐volcanic features. In addition, a new approach of neural network analysis on log data was applied to detect and quantitatively characterise hydrothermal mounds that are hard to interpret solely from seismic data. The volcanic activity started in the Middle Miocene and induced the development of extrusive volcanic mounds south of the NE‐SW trending, continuous strike‐slip fault zone (Hajdú Fault Zone). In the earliest Late Miocene (11.6–9.78 Ma), the style of magmatic activity changed resulting in emplacement of intrusions and development of hydrothermal mounds. Sill emplacement occurred from south‐east to north‐west based on primary flow‐emplacement structures. The time of sill emplacement and the development of hydrothermal mounds can be bracketed by onlapped forced folds and mounds. This time coincided with the acceleration of sedimentation producing poorly consolidated, water‐saturated sediments preventing magma from flowing to the paleosurface. The change in extensional direction resulted in change in fault pattern, thus the formerly continuous basin‐bounding strike‐slip fault became segmented which could facilitate the magma flow toward the basin centre.     

Volcanic style in the Strand Fiord Formation (Upper Cretaceous), Axel Heiberg Island, Canadian Arctic Archipelago

The Strand Fiord Formation is a volcanic unit of early Late Cretaceous age which outcrops on west-central and northwestern Axel Heiberg Island in the Canadian Arctic Archipelago. The formation is part of the thick Sverdrup Basin succession and immediately precedes the final basin foundering event. The Strand Fiord volcanics are encased in marine strata and thin southward from a maximum thickness of 789+ m on northwestern Axel Heiberg to a zero edge near the southern shore of the island. Tholeiitic icelandite flows are the main constituent of the formation with volcaniclastic conglomerates, sandstones, mudrocks and rare coal seams also being present. The lava flows range in thickness from 6 to 60 m and subaerial flows predominate. Both pahoehoe and aa lava types are common and the volcanic pile accumulated mostly by the quiet effusion of lavas. The volcaniclastic lithologies become more common near the southern and eastern edges of the formation and represent lahars and beach to shallow marine reworked deposits. The Strand Fiord volcanics are interpreted to represent the cratonward extension of the Alpha Ridge, a volcanic ridge that was active during the formation of the Amerasian Basin.     

The architecture of submarine monogenetic volcanoes – insights from 3D seismic data

Many prospective sedimentary basins contain a variety of extrusive volcanic products that are ultimately sourced from volcanoes. However, seismic reflection‐based studies of magmatic rift basins have tended to focus on the underlying magma plumbing system, meaning that the seismic characteristics of volcanoes are not well understood. Additionally, volcanoes have similar morphologies to hydrothermal vents, which are also linked to underlying magmatic intrusions. In this study, we use high resolution 3D seismic and well data from the Bass Basin, offshore southern Australia, to document 34 cone‐ and crater‐type vents of Miocene age. The vents overlie magmatic intrusions and have seismic properties indicative of a volcanic origin: their moderate–high amplitude upper reflections and zones of “wash‐out” and velocity pull‐up beneath. The internal reflections of the vents are similar to those found in lava deltas, suggesting they are composed of volcaniclastic material. This interpretation is corroborated by data from exploration wells which penetrated the flanks of several vents. We infer that the vents we describe are composed of hyaloclastite and pyroclasts produced during submarine volcanic eruptions. The morphology of the vents is typical of monogenetic volcanoes, consistent with the onshore record of volcanism on the southern Australian margin. Based on temporal, spatial and volumetric relationships, we propose that submarine volcanoes can evolve from maars to tuff cones as a result of varying magma‐water interaction efficiency. The morphologies of the volcanoes and their links to the underlying feeder systems are superficially similar to hydrothermal vents. This highlights the need for careful seismic interpretation and characterization of vent structures linked to magmatic intrusions within sedimentary basins.     

A simplified stress analysis of rising salt domes

We use a simple analytical model to estimate the stress field in density‐driven, rising salt domes and adjacent sediments, and to describe the evolution of these domes. We show that the pressure exerted by the salt pushing out against its wall rocks (the salt pressure) decreases linearly up the flank of the dome, but is always greater than the overburden stress. In fact, the salt pressure normal to the dome boundary is everywhere the maximum principal stress, whereas the hoop stress parallel to the circumference of the dome is the minimum stress. In addition, we quantitatively describe the critical stages of salt dome evolution (initiation, upbuilding, and downbuilding), relating these stages to sedimentation rate and basin thickness. This analysis also shows that even the highest sedimentation rates are unlikely to accumulate enough sediments to bury downbuilding domes as long as the salt supply is unrestricted. Despite the simplicity of the model, its predictions are in good agreement with field observations near salt domes. Overall, our analytical model can provide critical insight into the stress field perturbation in and near rising salt domes and can be used to assess the accuracy of numerical models and field measurements near these domes.     

Crustal thickening and crustal extension as controls on the evolution of the drainage network of the central Swiss Alps between 30 Ma and the present: constraints from the stratigraphy of the North Alpine Foreland Basin and the structural evolution of the Alps

The combined information about sedimentary petrography from the North Alpine Foreland Basin and structural geology from the Alps allows a qualitative reconstruction of the drainage network of the central Swiss Alps between 30 Ma and the present. This study suggests that crustal thickening and crustal thinning significantly controlled the location of the drainage divide. It also reveals the possible controls of crustal thickening/thinning on the change of the orientation of the drainage network from across-strike between 30 and 14 Ma to along-strike thereafter. Initial crustal thickening in the rear of the wedge is considered to have formed the drainage divide between north and south at 30 Ma. Because the location of crustal thickening shifted from east to west between ≈30–20 Ma, the catchment areas of the eastern dispersal systems reached further south than those of the western Alpine palaeorivers for the same time slice. Similarly, the same crustal dynamics appear to have controlled two phases of denudation that are reflected in the Molasse Basin by petrographic trends. Uplift in the rear of the wedge caused the Alpine palaeorivers to expand further southward. This is reflected in the foreland basin by increasing admixture of detritus from structurally higher units. However, tectonic quiescence in the rear of the wedge allowed the Alpine palaeorivers to cut down into the Alpine edifice, resulting in an increase of detritus from structurally lower units. Whereas uplift in the rear of the wedge was responsible for initiation of the Alpine drainage systems, underplating of the external massifs some 50 km further north is thought to have caused along-strike deviation of the major Alpine palaeorivers. Besides crustal thickening, extension in the rear of the wedge appears to have significantly controlled the evolution of the drainage network of the western Swiss Alps. Slip along the Simplon detachment fault exposed the core of the Lepontine dome, and caused a 50-km-northward shift of the drainage divide.     

Diatom-inferred palaeoenvironmental changes of a Pliocene lake disturbed by volcanic activity

Lake development in geologically active basins is a complex phenomenon as shown by the bed of the Pliocene Tlaxcala paleolake disturbed by volcanic eruptions in central Mexico. A 5-m layered freshwater diatomite outcrop section shows volcanic ashes throughout a bottom Stephanodiscus zone up to a more recent Aulacoseira zone. A 0.5-m transition region begins with an abrupt decrease in volcanic inputs and ends with a shift to Aulacoseira solida (Eulenstein) Krammer. Sediment analysis points to a shallow eutrophic lake in the bottom zone undergoing a transition to higher water volume leading to a threshold depth for the species shift. The lake was apparently deeper and mesotrophic thereafter. A common interpretation is to attribute such a volume change to a moisture increase. However, evidence of geological activity, mainly volcanism and faulting, suggests that it could have also been rooted in geomorphological changes of the lake basin.     

西南极利文斯顿岛百耳斯半岛火山岩的同位素年龄

以Ｋ－Ａｒ稀释法和４０Ａｒ／３９Ａｒ阶段加热法对利文斯顿岛百尔斯半岛的熔岩和次火山岩测定了同位素年龄。新的年龄数据表明,该区火山活动在晚侏罗纪末即已开始,一直持续到晚白垩世,存在着三期火山活动的产物。侏罗纪末～早白垩世初的火山活动除有熔岩（１４６Ｍａ,１３７Ｍａ）喷出外,还发育有辉长岩侵入体（１３５Ｍａ）和辉绿岩岩墙（１２９Ｍａ）等次火山岩。安山岩（１１４Ｍａ）和石英斑岩（９４Ｍａ）则是早白垩世火山活动的结果。半岛东部具沉积夹层的玄武岩（７１Ｍａ）是区域上晚白垩世～早第三纪火山活动的代表。