Characteristic features of salt tectonics offshore North Sinai, Egypt

Salt tectonic along offshore North Sinai was studied using seismic reflection data. The study revealed and identified various types of salt tectonics and structures in the study area. The triggering mechanism of salt tectonics was attributed to the pressure regime initiated from overloading sediments on the Messinian evaporites. The sediment load of 3,000?m exceeds the critical load (more than 1,000?m) and hence creates a pressure zone. The salt-generated structures resulted from thin-skinned extension that is driven by gravity gliding of the overloading sediments above the Messinian evaporite boundary, which acts as a detachment layer. These structures comprise normal growth faults and keystone grabens, trending roughly perpendicular to the slope of the continental margin. Salt tectonics in the study area were also triggered by the deformation of the movement of evaporite layer that causes stretching and fragmentation of the evaporite layer. Moving salt layer took place laterally and vertically, causing lateral and vertical pressures inside the Pliocene sediments. These movements of sediments led to the formation of salt rollers, salt weld, salt diapirs, rollover structures, and fault blocks. The interpretation of seismic data illustrates that the evaporite layer was switched off between the famous reflector M at its top and another reflector N at its base. M reflector is present and can be traced across the whole study area, while the N reflector pinched out in some parts of the study area.     

Delta-fed turbidites in a lacustrine rift basin: the Eocene Dongying depression,Bohai Bay Basin,East China

Sediment avalanche from delta ramp is one of the significant development mechanisms for a turbidite system in a lacustrine basin. To advance our understanding of deep-water sedimentary processes in a lacustrine delta ramp, delta-fed turbidites in the Eocene Dongying depression of the Bohai Bay Basin were studied using core data, 3-D seismic data and well log data. Sandy debris flows, muddy debris flows, mud flows, turbidity currents, slides, sandy slumps and muddy slumps were interpreted based on the identification of lithofacies. Data indicates that deep-water sedimentary processes in the study area were dominated by debris flows and slumps, which accounted for ～68% and 25% (in thickness) of total gravity flow deposits, respectively; turbidity-current deposits only accounted for ～5%. Mapping of turbidites showed that most were deposited after short-distance transportation (<20 km), restricted by the scale of deep-water areas and local topography. Channels, depositional lobes, debris flow tongues, muddy turbiditic sheets, slides and slumps were identified in a delta-fed ramp system. Slides and slumps were dominant at the base of slopes or at the hanging walls of growth faults with strong tectonic activity. Channels and depositional lobes developed in gentle, low-lying areas, where sediments were transported longer distances. Sand-rich sediment supply, short-distance transportation and local topography were crucial factors that controlled sedimentation of this ramp system. Channels generally lacked levees and only produced scattered sandstones because of possible hydroplaning of debris flow and unstable waterways. In addition to lobes, debris flow tongues could also be developed in front of channels. These findings have significant implications for hydrocarbon exploration of deep-water sandstone fed by deltas in a lacustrine basin.     

Depositional processes and basin analysis of Messinian evaporites in Cyprus

Messinian evaporites in Cyprus resulted from the interplay of Mediterranean-wide and eustatic sea-level changes and local tectonics, in an inferred above-subduction zone setting. Distinctive Tortonian-early Messinian pre-evaporitic facies include diatomaceous marls and microbial carbonates, overlain by a variety of gypsum facies and then by lagoonal-lacustrine deposits and local palaeosols. Facies analysis and comparisons allow construction of a simple model, in which evaporites formed in semi-isolated small basins not far below global eustatic sea-level. Coarsely crystalline gypsum formedin situ along the margins of small basins and within shallow-water lagoons (< 10 m deep); this comprised common banded-stacked (i.e. layered) selenite, swallowtail selenite, botryoidal selenite and sugary-bedded selenite. Fine-grained gypsum precipitated widely and was reworked into basinal areas (< 70 m deep) by weak traction currents and low-density turbidity currents. Shallow-water derived selenite was also reworked basinwards by high-density turbidity currents and debris flows. Slumps indicate tectonic instability. More detailed basin analysis can be achieved by study of individual sub-basins. In the Polemi sub-basin in the west, a Lower Unit (up to 60 m thick) comprises basinal gypsum, interbedded with gypsum turbidites and mass flow deposits, with slumps. This is overlain by an extensive mega-rudite (up to 20 m thick) including up to metre-sized clasts of marginal gypsum facies. Above, the Upper Unit (up to 70 m thick) includes shallow-water gypsum (e.g. swallowtails), marl and minor microbial carbonates. The Pissouri sub-basin in the south-west exposes marginal facies of the Upper Unit, including deltaic elastics and palaeosols. The Maroni sub-basin in the south exhibits a basinal lower gypsum unit, with laterally equivalent marginal facies (up to 50 m thick), overlain by an extensive mega-rudite (up to 20 m thick). Finally, the Mesaoria subbasin in the north exposes relatively marginal gypsum facies in an unstable tectonic setting. Formation of the Polemi, Pissouri and Mesaoria gypsum sub-basins relates to coeval extensional faulting and graben development. Evaporites in south Cyprus (Maroni sub-basin) formed in elongate basins between former compressional lineaments created by localized Early Miocene thrusting. In the sub-basins of west, south-west and south Cyprus, large-scale slumping of marginal gypsum facies took place towards depocentres (to form megarudite debris flows), triggered by one or several phases of extensional faulting.     

Facies analysis of the Trypali carbonate unit (Upper Triassic) in central-western Crete (Greece): an evaporite formation transformed into solution-collapse breccias

The Trypali carbonate unit (Upper Triassic), which crops out mainly in central‐western Crete, occurs between the parautochthonous series (Plattenkalk or Talea Ori‐Ida series, e.g. metamorphic Ionian series) and the Tripolis nappe (comprising the Tripolis carbonate series and including a basal Phyllite–Quartzite unit). It consists of interbedded dolomitic layers, represented principally by algally laminated peloidal mudstones, foraminiferal, peloidal and ooidal grainstones, as well as by fine‐grained detrital carbonate layers, in which coarse baroque dolomite crystals and dolomite nodules are dispersed. Baroque dolomite is present as pseudomorphs after evaporite crystals (nodules and rosettes), which grew penecontemporaneously by displacement and/or replacement of the host sediments (sabkha diagenesis). However, portions of the evaporites show evidence of resedimentation. Pre‐existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal‐shaped hoppers, as well as of anhydrite nodules and rosettes (salt crusts). All microfacies are characteristic of peritidal depositional environments, such as sabkhas, tidal flats, shallow hypersaline lagoons, tidal bars and/or tidal channels. Along most horizons, the Trypali unit is strongly brecciated. These breccias are of solution‐collapse origin, forming after the removal of evaporite beds. Evaporite‐related diagenetic fabrics show that there was extensive dissolution and replacement of pre‐existing evaporites, which resulted in solution‐collapse of the carbonate beds. Evaporite replacement fabrics, including calcitized and silicified evaporite crystals, are present in cements in the carbonate breccias. Brecciation was a multistage process; it started in the Triassic, but was most active in the Tertiary, in association with uplift and ground‐water flow (telogenetic alteration). During late diagenesis, in zones of intense evaporite leaching and brecciation, solution‐collapse breccias were transformed to rauhwackes. The Trypali carbonate breccias (Trypali unit) are lithologically and texturally similar to the Triassic solution‐collapse breccias of the Ionian zone (continental Greece). The evaporites probably represent a major diapiric injection along the base of the parautochthonous series (metamorphic Ionian series) and also along the overthrust surface separating the parautochthonous series from the Tripolis nappe (Phyllite–Quartzite and Tripolis series). The injected evaporites were subsequently transformed into solution‐collapse breccias.     

The Messinian Sicilian stratigraphy revisited: new insights for the Messinian salinity crisis

Controversies around the Messinian salinity crisis (MSC) are because of the difficulties in establishing genetic and stratigraphic relationships between its deep and shallow‐water record. Actually, the Sicilian foreland basin shows both shallow and deep‐water Messinian records, thus offering the chance to reconstruct comprehensive MSC scenarios. The Lower Gypsum of Sicily comprises primary and resedimented evaporites separated in space and time by the intra‐Messinian unconformity. A composite unit including halite, resedimented gypsum and Calcare di Base accumulated between 5.6 and 5.55 Ma in the main depocentres; it records the acme of the Messinian Salinity Crisis during a tectonic phase coupled with sea‐level falls at glacials TG14‐TG12. These deposits fully post‐date primary gypsum, which precipitated in shallow‐water wedge‐top and foreland ramp basins between 5.96 and 5.6 Ma. This new stratigraphic framework results in a three‐stage MSC scenario characterized by different primary evaporite associations: selenite in the first and third stages, carbonate, halite and potash salt in the second one associated with hybrid resedimented evaporites.     

Distribution and characteristics of landslides induced by the Iwate–Miyagi Nairiku Earthquake in 2008 in Tohoku District, Northeast Japan

The Iwate–Miyagi Nairiku Earthquake in 2008, whose seismic intensity was M. 7.2 in Japan Meteorological Agency (JMA) scale, induced innumerable landslides on the southern flank of Mt. Kurikoma volcano allocated along the Ou Backbone Range in Northeast Japan. Most landslides are detected in a hanging wall side of the seismic fault. Those landslides are classified into five types: deep-seated slide, debris slide, shallow debris slide, secondary shallow debris slide, and debris flow. Most common landslide types induced by the earthquake are shallow debris slides and subsequent debris flows. They are intensively distributed along steep gorges incising a volcanic skirt of Mt. Kurikoma, consisting of welded ignimbrite of the Pleistocene age. Debris flows are also distributed even along gentle river floors in the southern lower flank of the volcano. The area of densely distributed debris slides, shallow debris slides, and debris flows is concordant with that of severe seismic tremor. Thus, genetic processes of landslides induced by the Iwate–Miyagi Nairiku Earthquake in 2008 are attributed to multiple causative factors such as geology, topography, and seismic force.     

Deep-water clastic evaporites deposition in the Messinian Adriatic foredeep (northern Apennines,Italy): did the Mediterranean ever dry out?

A new genetic facies model for deep-water clastic evaporites is presented, based on work carried out on the Messinian Gessoso-solfifera Formation of the northern Apennines during the last 15 years. This model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts. The model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites, representing the most common evaporite facies of the northern Apennines, and the definition of the genetic and stratigraphic relationship with primary shallow-water evaporites formed and preserved in marginal settings. Due to the severe recrystallization processes usually affecting these deposits, petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity-driven deposits ranging from debrisflow to low-density turbidites. Almost all the laminar ‘balatino’ gypsum, previously considered a deep-water primary deposit, is here reinterpreted as the fine-grained product of high to low-density gravity flows. Facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the Apennine foredeep basin. The Messinian Salinity Crisis occurred during an intense phase of geodynamic reorganization of the Mediterranean area that also produced the fragmentation of the former Miocene Apennine foredeep basin. In this area, primary shallow-water evaporites equivalent to the Mediterranean Lower Evaporites, apparently only formed in semi-closed thrust-top basins like the Vena del Gesso Basin. The subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins. Vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the Messinian tectonic evolution of the Apennine thrust belt and related exhumation–erosional processes. The facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts; the importance of clastic evaporites deposits has been overlooked in the study of other Mediterranean areas. Based on the Apennine basins experience, it is suggested here that evaporites diffused into the deeper portions of the Mediterranean basin may consist mainly of deep-water resedimented deposits rather than shallow-water to supratidal primary evaporites indicative of a complete basin desiccation.     

High-resolution (2–7 kHz) acoustic and geometric characters of submarine creep deposits in the South Korea Plateau, East Sea

A synthesis of high-resolution (Chirp, 2–7 kHz) seismic profiles in the South Korea Plateau reveals that large masses of wavy stratified sediment (≈60–90 m thick) cover broad, gently sloping (<0·5°) ridges in water depths of 1000–2000 m. The wavy stratified sediment (WSS) is characterized by wavy (0·2–5 km in wavelength and <15 m in relief), continuous reflective layers with a basal deformed zone that overlies undeformed, strong reflectors. The WSS exhibits systematic variation in wave dimensions and thickness of internal reflective layers with changes in slope gradient. The troughs of the waves are commonly associated with internal growth faults, and wave amplitude generally increases with subbottom depth. On steep slopes around the ridges, the WSS masses are bounded downslope by slide and slump deposits including slightly translated or rotated WSS blocks. The acoustic and geometric characters, and association with downslope slides and slumps on the steeper slopes, suggest that the WSS masses were most probably formed by slow creep movement before slope failure. In the absence of significant sediment input to the South Korea Plateau, the deep (1000–2000 m in water depth) mass movements were probably triggered by earthquakes that have occurred frequently in this region. Some slightly displaced, intact WSS blocks in the associated slides and slumps downslope reflect a progressive evolution from submarine creep into slide and slump.     

The Casanova Complex of the Northern Apennines: A mélange formed on a distal passive continental margin

The Cretaceous-Palaeocene Casanova Complex occurs in two thrust sheets of the eugeosynclinal Ligurids of the Northern Apennines. It is a sedimentary mélange with ophiolitic and quartzose turbidites or limestone-shale olistostrome (submarine debris flows) as matrix. Exotic blocks of ophiolite and granite, serpentinite breccias and lenticular ophiolitic breccias and olistostromes contribute to the mélange character of the complex. Deformational structures include soft-sediment slump folds (indicating a SW-dipping palaeoslope) and boudins, a gradational slumped top to the mélange, small-scale faults in chert blocks and deformation associated with the emplacement of the exotic slide blocks. The blocks were shed as rotational slides from submarine fault scarps and are surrounded by haloes of debris created by submarine weathering. The stacking pattern of the blocks, with the originally stratigraphically highest ophiolite lithologies lowest in the pile of blocks, is explained by a diverticulation model with progressively deeper erosion. Mechanical analysis shows that the blocks were stable when partly exposed resting on a soft sediment substratum. Criteria which distinguish the Casanova Complex from a tectonic mélange, and which may be of value in other mélanges, are discussed. Previous interpretations of the complex as a precursor olistostrome to northeastward nappe emplacement (the Bracco ridge model) are rejected. The mélange is believed to have formed on ocean crust as a result of turbidite and debris flow sedimentation, soft sediment deformation, block faulting, gravity sliding and submarine erosion at the distal edge of a uniformly SW-dipping continental margin.     

Three-dimensional architecture of submarine slide surfaces and associated soft-sediment deformation in the Lutetian Sobrarbe deltaic complex (Ainsa, Spanish Pyrenees)

Five successive fossil submarine slides have been mapped and described in the Sobrarbe deltaic complex (Ainsa Basin, Spanish Pyrenees). These slides affect and remove up to 15% of the delta front. The head of the scar surfaces is recognized clearly in the field due to the angular unconformity between the infilling sediments and the underlying layers. Most of the slide scarps trend 55°N with 20° to 40° dips indicating north-westward sliding. Downslope, traces of the sliding surfaces parallel stratification. However, these surfaces can be identified by displaced masses, resedimented sandstones and soft-sediment deformation features such as metre-scale half-grabens, normal faults and tension cracks; all the surfaces indicate a sliding displacement toward the north-west. A three-dimensional model built from topographic data with Earth Vision^® software shows the architecture of the slide surfaces and provides an estimation of the volume of each sedimentary body within the limit of the studied area. This study also indicates that: (i) the sediments have been cut and carried away before their lithification; (ii) the sedimentation rate infilling a single slump scar is estimated to be about 8 m per 1000 years, i.e. 10 times higher than in the overall area of the Sobrarbe deltaic complex; (iii) each composite scar progressively develops and infills by retrogressive slumps; (iv) the successive slide surfaces stack vertically in a collapse complex structure and migrate downward to follow the sea-level drop between two successive collapse complex structures; (v) the development of the scars in the Sobrarbe delta is described from a seven-stage conceptual model starting with a regressive event; and (vi) the triggering of the Sobrarbe instabilities is controlled by high values of sedimentation rate, relative sea-level falls mainly controlled by tectonic uplift, and likely seismic activity.     

汶川8级地震地质灾害的类型及实例

龙门山地区活动断裂右旋斜冲运动与汶川8级强烈地震存在成因联系。汶川8级地震造成了惨重的人员伤亡和巨大的财产损失,地震地质灾害主要类型有地震灾害、地震触发地质灾害和地震引发地质灾害隐患。严重地震灾害包括房屋倒塌与部分坍塌、房屋平移、房屋倾斜变形、墙体破裂与结构破坏、桥梁垮塌等。地震触发严重地质灾害包括山体滑坡、山体滑塌、岩块崩塌,局部产生泥石流与沙土液化。地震引发地质灾害隐患包括潜在滑坡、不稳定边坡与滑塌隐患、危岩体与崩塌隐患及泥石流灾害隐患。地震地质灾害分布与活动断层存在密切关系,沿北川映秀断裂地震地质灾害最为严重,沿汉旺漩口断裂、茂县汶川断裂、青川断裂地震地质灾害也比较严重。在活动断裂之间相对稳定地块远离活动断裂超过3~5 km,地震灾害和地震触发地质灾害显著减轻。     

Evaporite karst in Sicily

Karst areas are distributed over most of Sicily. The most widespread karst rocks are carbonates, particularly limestones, but karst phenomena can also be seen in evaporites and particularly in salt mines. This report provides an overview of evaporite karst in Sicily, along with a “case history” that shows some of the evaporite karst risks to the environment. In the centre and south of Sicily, a thick sequence of Messinian evaporite rocks are subject to dissolution from meteoric and formation waters. In areas where potassium salts and rock salts are being mined, some geomorphologic changes result from the drilling of boreholes and the collapse of underground mines, thus lowering or collapsing the land surface. An example is the old salt mine “Muti-Coffari”, situated in the commune of Cammarata, where there is a modification of the surface flow of the River Platani. Meteoric waters and runoff flow down through a borehole, enter the underground mine cavity and dissolve the salts, and then the resulting brine flows into a branch of the river, making it salty. Field investigations showed the presence of salt along the edges and on the bed of the stream where it comes out of the cave. Therefore, interventions for risk mitigation are necessary since the old mine constitutes a serious danger for damage or collapse of nearby infrastructures, and can lead to degradation of the river ecosystem and the natural environment.     

Characteristics and remediation of a landslide complex triggered by the 2008 Wenchuan, China earthquake-case from Yingxiu near the earthquake epicenter

During the 2008 Wenchuan earthquake, the river valley from Yingxiu to Wenchuan experienced numerous landslides and became a prominent area of landslide complexes. The present large landslide complex near the earthquake epicenter consisted of Laohuzui slide 1, Laohuzui slide 2 and Douyaping slide. The scale, geology, morphology, sliding process, and failure mechanism of the landslide complex are analyzed by means of field investigation, aerial photograph and stereographic projection technique. Characteristics of these three slides including seismic response of slope, landslide debris, damage and potential failure are discussed: the convex slope and the upslope of fractured granitic rock at high altitude are highly prone to landsliding under earthquake; the high source altitude and long travel path determine grain sizes and the deposit angle of the slide debris; the landslide complex completely buries the G213 roadway and dams up the Minjiang River in these sections; after the earthquake, rainfall, aftershocks and river erosion may retrigger new failures, such as retrogressive slide of weathered fractured rock, colluvial landslide, debris flow, embankment failure and rockfall. The following are presented as suggested remedial measures to protect the roadway and stabilize the slope: the removing and trenching, protective concrete/rock blocks against erosion, retaining structure, rockfall stopping wall, rockfall restraining net, rock bolt, and the planting of vegetation.     

The role of E–W basement faults in the Mesozoic geodynamic evolution of the Gafsa and Chotts basins,south-central Tunisia

The Gafsa and Chotts intracratonic basins in south-central Tunisia are transitional zones between the Atlasic domain to the north and the Saharan platform to the south. The principal aim of this paper is to unravel the geodynamic evolution of these basins following an integrated approach including seismic, well log and gravity data. These data are used to highlight the tectonic control on the deposition of Jurassic and Lower Cretaceous series and to discuss the role of the main faults that controlled the basin architecture and Cretaceous–Tertiary inversion. The horizontal gravity gradient map of the study area highlights the pattern of discontinuities within the two basins and reveals the presence of deep E–W basement faults. Primary attention is given to the role played by the E–W faults system and that of the NW–SE Gafsa fault which was previously considered active since the Jurassic. Facies and thickness analyses based on new seismic interpretation and well data suggest that the E–W-oriented faults controlled the subsidence distribution especially during the Jurassic. The NW–SE faults seem to be key structures that controlled the basins paleogeography during Late Cretaceous–Cenozoic time. The upper Triassic evaporite bodies, which locally outline the main NW–SE Gafsa fault, are regarded as intrusive salt bodies rather than early diapiric extrusions as previously interpreted since they are rare and occurred only along main strike-slip faults. In addition, seismic lines show that Triassic rocks are deep and do not exhibit true diapiric features.     

Tectonic and sedimentary evolution of the Lower Pliocene Periadriatic foredeep in Central Italy

The Periadriatic foredeep (Italy) was generated by Neogene downbending of the Adria Plate under the Apennine Chain. The basin is filled with Plio-Pleistocene siliciclastic turbidites. Its substratum consists of the carbonate succession of the southwestern Adria Plate margin. The influence of the basin’s morphology on sedimentation and subsequent tectonic evolution is investigated in the Abruzzo sector of the foredeep (Cellino Basin). The substratum is composed of Messinian evaporites that dip towards the Apennines (W). A NNW component along the depocentral axis is divided into four blocks with different depths. The substratum was also affected by a Messinian extensional fault system, not involving the overlying Pliocene sequence. This morphology controlled the distribution of the turbidites in the lower part of the Cellino Basin. The Plio-Pleistocene compressional deformation of the foredeep produced an inner complex structure (Internal Structure), involving the foredeep substratum and an outer imbricate thrust system (Coastal Structure), detached over the faulted Messinian evaporites. This thrust system is parallel to the extensional faults, suggesting a strong influence of the substratum morphology on the development of the compressional structures. The overall structural setting was validated with a balanced cross-section. Out-of-sequence thrusting and non-coeval deformation within each thrust sheet characterize the local tectonic history.     

特提斯域的密西西比河谷型(MVT)铅锌矿床

特提斯域是全球最重要的密西西比河谷型(MVT)铅锌矿床富集区,矿床广泛地分布在陆-陆碰撞造山带两侧的陆块上,就位于碰撞形成的褶皱-逆冲带和前陆带内,成矿通常发生在碰撞挤压作用晚期或之后、区域处于走滑或伸展的阶段。矿床、矿体的就位与张性断裂密切相关,主要控矿因素有蒸发盐底辟构造、碳酸盐岩溶蚀垮塌构造、蒸发盐溶蚀垮塌构造、高孔隙度白云岩、含重晶石地层等。油气流体在许多矿床和矿集区出现,其主要通过与硫酸盐发生反应为铅锌成矿提供还原硫。陆-陆碰撞的大地构造环境、大量的蒸发盐、丰富的油气流体是特提斯域富集MVT矿床的重要原因,域内寻找MVT矿床的潜力巨大。     

Messinian deposits and erosion in northern Tunisia: inferences on Strait of Sicily during the Messinian Salinity Crisis

Outcrops, offshore wells, electric logs and seismic profiles from northern Tunisia provide an opportunity to decipher the Messinian Salinity Crisis in the Strait of Sicily. Messinian deposits (including gypsum beds) near the Tellian Range reveal two successive subaerial erosional surfaces overlain by breccias and marine Zanclean clays, respectively. In the Gulf of Tunis, Messinian thick evaporites (mostly halite) are strongly eroded by a fluvial canyon infilled with Zanclean clays. The first erosional phase is referred to the intra-Messinian tectonic phase and is analogous to that found in Sicily. The second phase corresponds to the Messinian Erosional Surface that postdates the marginal evaporites, to which the entire Sicilian evaporitic series must refer. The Western and Eastern Mediterranean basins were separated during deposition of the central evaporites.     

GIS and ANN coupling model: an innovative approach to evaluate vulnerability of karst water inrush in coalmines of north China

Evaporites, including rock salt (halite) and gypsum (or anhydrite), are the most soluble among common rocks; they dissolve readily to form the same types of karst features that commonly are found in limestones and dolomites. Evaporites are present in 32 of the 48 contiguous states in USA, and they underlie about 40% of the land area. Typical evaporite-karst features observed in outcrops include sinkholes, caves, disappearing streams, and springs, whereas other evidence of active evaporite karst includes surface-collapse structures and saline springs or saline plumes that result from salt dissolution. Many evaporites also contain evidence of paleokarst, such as dissolution breccias, breccia pipes, slumped beds, and collapse structures. All these natural karst phenomena can be sources of engineering or environmental problems. Dangerous sinkholes and caves can form rapidly in evaporite rocks, or pre-existing karst features can be reactivated and open up (collapse) under certain hydrologic conditions or when the land is put to new uses. Many karst features also propagate upward through overlying surficial deposits. Human activities also have caused development of evaporite karst, primarily in salt deposits. Boreholes (petroleum tests or solution-mining operations) or underground mines may enable unsaturated water to flow through or against salt deposits, either intentionally or accidentally, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause land subsidence and/or catastrophic collapse. Evaporite karst, natural and human-induced, is far more prevalent than is commonly believed.     

Large,prehistoric clay slides revealed in road excavations in Buvika,mid-Norway

Several excavated sections along new roads in Buvika, mid-Norway, display records of large, prehistoric clay slides. Slightly undulating but otherwise intact laminated clay, with minor amounts of unsorted or sorted coarser-grained debris, appears in the sections. Folding, shearing and internal slide surfaces have also been recorded, and the deposits are interpreted as slide debris. Slide-scar morphologies are diffuse or absent for most of these deposits, and the inferred slide mechanism is translation and rotation of blocks on a thin layer of remoulded quick clay. One section cuts through a slide block inside a classic, morphologically well-defined quick-clay slide scar. Varying results from ¹⁴C-datings of organic material in the deposits give only maximum ages of the slide events. The style of sliding reflected in several sections differs from slide events in the area known from historical records. These younger slide events seem to be characterised by collapse and remoulding of thicker quick-clay layers. The present study of slide deposits gives information on processes and deformation history of the slides. It is suggested that the gradual formation of quick clay has an important impact on long-term landscape degradation and on the character of the present day landscape. There may be far more signs of slide activity in Norwegian valleys than solely indicated by slide scars and also in the relatively high-lying parts of the valley fill, such as interfluve areas.     

伸展构造及其反转作用

伸展构造的演化主要发生在三个阶段：大陆裂谷阶段、大陆初始漂移和主要漂移阶段。伸展构造发育在不同构造层次：（１）在上地壳中，伸展构造呈现为位于拆离带之上脆性岩块的伸展，其周边为铲式断层、坡坪式断层或多米诺式断层所限；（２）在中地壳中，伸展构造呈现为变质透镜体，被不连续的韧性剪切带所分隔；（３）在下地壳中，伸展构造呈现为扩展变平的韧性流动和岩浆侵位。反转的伸展断层系常表现为冲断层、断弯背斜、生长断展褶皱、顶部塌陷地堑区的突隆构造和半地堑区的鱼叉构造等。