Slumping and shallow faulting related to the presence of salt on the Continental Slope and Rise off North Carolina

Seismic reflection profiles and long- and medium-range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the Continental Slope off North Carolina. Within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long. The slope above the scarp is characterized by a series of shallow rotational normal faults. The bottom below the scarp is furrowed by slide tracks, which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes.Extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion, which has fractured and oversteepened the slope leading to instability and failure. Sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process, in contrast to much of the surrounding slope area where few instability features were observed.     

Nature of the connection between the Northern and Southern Zechstein Basins across the Mid North Sea High

The seismic expression of a salt-filled channel which cuts across the Mid North Sea High in Quadrant 37 is described, with features interpreted as being produced by salt-edge dissolution forming both eastern and western margins of the channel. The apparent half-graben nature of the channel is shown to be only superficial, and due to complex faulting associated with, but not defining, its western margin. The shallower faulting here is a Mesozoic to early Tertiary growth fault related to local dissolution of Zechstein salt. The dissolution effect appears in turn to have been localized by the presence of a deeper fault that was already downthrown to the east in Zechstein times, when it seems to have limited the eastward progradation of Zechstein shelf carbonates and anhydrites, and had probably ceased to move significantly before the onset of the Late Cimmerian erosional phase. The origin of this arcuate fault is tentatively ascribed to subsidence around a granite batholith. Zechstein salt originally spread some distance to the east and west of the channel; it was dissolved from the edges inwards, mainly before the Late Cretaceous, giving rise to a thicker Mesozoic sequence on parts of the flanks of the channel than in the middle. Besides providing an interesting structural case history, the features described have implications regarding Zechstein sedimentation, reservoir potential, the tectonic history of the North Sea, and the nature of the Mid North Sea High itself.     

Mantle flow patterns at the Neyriz Paleo-spreading center, Iran

The mantle peridotites of Neyriz record two successive episodes of plastic deformations; the first one related to the igneous accretion of the lithosphere and the second one developed during the first stage of the emplacement of the peridotites. These two events have been distinguished on the basis of microstructural criteria. The diapiric pattern, particularly relevant to the mantle process beneath spreading ridges, features vertical flow lines and elliptic flow plane trajectories in a pipe and extends along the ridge axis about 5 km. These structures rotate to horizontal and diverge in every direction in a narrow transition zone, a few hundred meters thick, below the Moho discontinuity. Such a diapiric pattern has been recognized in a few places along the Neyriz paleo-ridge. A large amount of magma passed through these mantle diapirs that were probably the main zones feeding the overlying magma chamber. The most common pattern features very regular structures over several kilometers along the strike of the paleo-ridge: the flow plane dips away from the ridge axis, and the flow line is parallel to the spreading direction. This flow pattern is frozen during the gradual accretion of the lithospheric mantle away from the ridge in a steady-state spreading regime. A shear-sense inversion at just below the Moho is commonly observed, pointing to forced asthenospheric flow. The reconstructed orientation of the Neyriz paleo-spreading center is 105°, compatible with the geometry and orientation of harzburgite foliations and lineations and sheeted dikes.     

云南建水煌斑岩年代学和地球化学及其构造意义

华南大陆内部燕山期的岩浆事件可分为2期:燕山早期180~150 Ma,燕山晚期140~80 Ma。而处于扬子板块西南缘的滇黔贵等地,仅见有第二期晚白垩世中酸性侵入岩与相关成矿作用,缺乏第一期岩浆活动的时间记录。本文在滇东建水地区首次发现了麦地村煌斑岩,对其进行高精度LA-ICP-MS锆石U-Pb定年和主微量元素测试分析。结果显示煌斑岩的锆石U-Pb年龄为(158.4±2.4)Ma(MSWD=4.2),为晚侏罗世的产物,补充了该区较为空白的第一期岩浆热事件。麦地村煌斑岩不相容元素明显高于原始地幔,表现出不同程度的富集,大离子亲石元素Rb、Ba、Ce等富集程度不明显,高场强元素中U、Ta、Pb等表现出一定程度富集,LREE相对富集,HREE相对亏损,不具有Nb-Ta-Ti负异常的俯冲型配分曲线特征,具有明显的OIB型洋岛玄武岩稀土元素配分特征,为板内伸展环境下的产物。滇东地区发育中生代时期的NNE构造,早期NNE向褶皱叠加有后期同方向正断层。早期褶皱表现为SEE-NWW挤压应力作用下的构造特征,后期断层表现出同方向张应力作用下的特征。煌斑岩正是由于该区发生挤压之后伴随着一期应力松弛调整,伸展减压作用诱发地幔物质部分融熔,来自于深处的软流圈地幔流体与岩石圈地幔发生交代作用,形成富集型地幔,交代富集的岩石圈地幔局部受热、部分熔融,发生上升侵位。煌斑岩的侵位时间即是滇东地区中生代从挤压向伸展转换的起始时间。     

Hydrocarbon alteration in the bituminous salt of the Kłodawa Salt Dome (Central Poland)

The so called bituminous salts occurring in the Kłodawa dome, located in Central Poland, differ from the surrounding salts by their colour change from light to dark brown. This colour is associated with an extremely large amount of hydrocarbon, mainly located in the inclusions. The presence of numerous fluid inclusions has been documented in previous petrologic studies, distinguishing seven main types of fluid inclusion assemblages (FIA) in terms of size, shape as well as the ratio of filling material. However, four types of inclusions were selected in the current investigations according to their unusual optical behaviour. Raman micro-spectroscopy a modern, non-destructive method was used for investigating a single inclusion being a part of FIA. Presented in this paper Raman spectra revealed a unique pattern of bands characterizing the content of the inclusions. The hydrocarbons show a very complex character reflected in the appearance of a strong fluorescence background. A well-marked heterogeneity characterized the inclusions, by diversity in the intensity of the background and in the pattern of the bands characterizing the presence of certain components. One can distinguish the presence of carbonaceous matter showing the different degrees of order. The depth profile and the analysis of the various points of the inclusions indicate that the carbonaceous matter is not evenly distributed in the inclusions but forms a thin, disorganized film on their walls. This film was also found in sites where the inclusions are filled with brine. The certain characteristics associated with the presence of the incipient phase transformation of the organic matter, or slightly transformed organic matter and the lack of light hydrocarbons as well as a number of petrologic features of inclusions indicate that these salt rocks have been subdivided into thermal transformations, accompanied by the recrystallization of the halite and the escape of the more volatile compounds such as methane, ethane, etc.     

Carbon isotopic change at the base of the upper permian zechstein sequence

Fluctuation of the carbon isotope composition of marine carbonates has recently been developed as a powerful tool for the identification of ocean-wide anoxic conditions and changes in the world budget of carbon and oxygen. A change in δ¹³ from the normal marine values (0 to + 2%) to values highly enriched in ¹³C (+3·5 to 4·5%) is recorded at the base of the Zechstein sequence both in central Germany and northeastern England. The change occurred over a relatively short period of time indicating a rapid and pronounced change in the organic carbon/carbonate budget. Evidence from other Permian basins show similar highly enriched δ¹³C values. This change may correspond to that in carbon balance distinguished by the Garrels and Perry (1975) model and based on the change in sulphur isotopic composition during the Permian.     

尼日尔三角洲东部深水区构造特征及其形成机理

研究尼日尔三角洲东部深水区块发现,整个盆地从陆向洋具有3个大的构造分区：伸展拉张区、过渡区和挤压逆冲区。伸展区以大型同沉积断层伴生大量滚动背斜构造为特征,过渡区发育大量泥底辟构造,挤压区以复杂的逆冲叠瓦构造为主。通过分析形成机理,揭示东部深水转换带上M研究区构造特征,按构造的演化特征将该区构造分为泥底辟型、冲断-泥底辟混合型、逆冲型3种类型,提出研究区内的圈闭主要以构造-岩性圈闭为主,为尼日尔三角洲盆地深水勘探提供新的理论指导。     

Middle Proterozoic–early Palaeozoic evolution of central Baltoscandian intracratonic basins: evidence for asthenospheric diapirs

The three intracratonic sedimentary basins located in central Baltoscandinavia, namely the Bothnian Gulf basin, the Bothnian Sea basin and the Baltic basin, developed in response to Middle Proterozoic and Late Proterozoic tectonic events, separated in time by about 800 Ma. Only the Baltic basin was subsequently affected by Caledonian orogenesis and Mesozoic rifting. Crustal extension was minor or did not take place during the Proterozoic basin evolution phases. However, according to the Moho topography, crustal thinning did take place. This was probably a result of subcrustal magmatism. On a craton-wide scale, the ages of granitoids, which intruded during the Middle Proterozoic basin formation, generally decrease from east to west. This fact, combined with the evidence provided by mantle-derived flood basalt magmatism, points to a moving asthenospheric diapir as the cause for basin development. Asthenospheric upwelling was probably also responsible for the second, Late Proterozoic, basin evolution phase, as evidenced by the lack of crustal thinning and extension, and the occurrence of tholeiitic intrusions. In addition, a Late Proterozoic thermally induced palaeo-high, located at about the position of the intracratonic basins, is compatible with indications from glaciations. As the ages of Late Proterozoic intracratonic basins also decrease from east to west across the craton, the location of asthenospheric diapirism during this time interval was also moving. For the Fennoscandian lithosphere, the presence of fundamental lithospheric weakness zones (e.g. terrane boundaries) might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the Fennoscandian Shield. The spacing and size of the Proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated. Therefore, sublithospheric convective processes might be the cause for the asthenospheric upwellings. Such processes are related to Rayleigh–Taylor instabilities in the sublithospheric mantle. Emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere. Modelling results demonstrate that erosion of the surficial high, succeeded by cooling of the lithosphere, can explain the accumulation of early Palaeozoic sediments in the Bothnian Sea basin, taking into account post-Ordovician vertical and lateral erosion of the basin fill.     

Salt diapirs without well defined boundaries – a feasibility study of semi‐automatic detection

Prestack depth imaging of seismic data in complex areas such as salt structures requires extensive velocity model updating. In many cases, salt boundaries can be difficult to identify due to lack of seismic reflectivity. Traditional amplitude based segmentation methods do not properly tackle this problem, resulting in extensive manual editing. This paper presents a selection of seismic attributes that can reveal texture differences between the salt diapirs and the surrounding geology as opposed to amplitude‐sensitive attributes that are used in case of well defined boundaries. The approach consists of first extracting selected texture attributes, then using these attributes to train a classifier to estimate the probability that each pixel in the data set belongs to one of the following classes: near‐horizontal layering, highly‐dipping areas and the inside of the salt that appears more like a low amplitude area with small variations in texture. To find the border between the inside of the salt and the highly‐dipping surroundings, the posterior probability of the class salt is input to a graph‐cut algorithm that produces a smooth, continuous border. An in‐line seismic section and a timeslice from a 3D North Sea data set were employed to test the proposed approach. Comparisons between the automatically segmented salt contours and the corresponding contours as provided by an experienced interpreter showed a high degree of similarity.     

Catastrophic and dangerous inflows to salt mines in Poland as related to the origin of water determined by isotope methods

 Tritium, ¹⁴C, δ¹³C, δ¹⁸O and δD measurements indicated as early as 1973 the existence of inflows of modern meteoric water to the Wapno salt mine in a Zechstein diapir. In spite of these early warnings, the continuation of improper exploitation led, in 1977, to sudden flooding of the mine followed by catastrophic land subsidence. The lesson learned from that catastrophe, as well as the results of isotope investigations performed in the Inowrocław salt mine, led to the decision to flood the mine artificially in order to avoid a similar land subsidence. The Kłodawa mine was not regarded to be in danger of flooding due to a thick clay cap. In fact, a large number of usually short-lasting water occurrences had the isotopic composition characteristic for evaporated ocean water. However, since 1956 an inflow has existed with δ¹⁸O and δD values close to that of pre-Quaternary saline waters and brines in the Mesozoic formations adjacent to the diapir. Two other inflows have recently occurred with the initial δ¹⁸O and δD values of modern waters. As a consequence, the mine is regarded to be in danger, and the exploitation of salt in the areas of inflows has been stopped. The Wieliczka mine, southern Poland, exploits Miocene salts overthrusted together with the Carpathian flysch from the south. The most dangerous and catastrophic inflows were caused by human errors. Isotope data show the water to be of glacial or Holocene age stored in Tertiary, slightly cemented rocks of low permeability, which neighbor the mine from the north. Owing to specific geology, the mine has survived for a long time, in spite of relatively large and long-lasting inflows. However, its existence is in permanent danger. Received: 7 March 1997 · Accepted: 17 November 1998