The Levantine Basin—crustal structure and origin

The origin of the Levantine Basin in the Southeastern Mediterranean Sea is related to the opening of the Neo-Tethys. The nature of its crust has been debated for decades. Therefore, we conducted a geophysical experiment in the Levantine Basin. We recorded two refraction seismic lines with 19 and 20 ocean bottom hydrophones, respectively, and developed velocity models. Additional seismic reflection data yield structural information about the upper layers in the first few kilometers. The crystalline basement in the Levantine Basin consists of two layers with a P-wave velocity of 6.0–6.4 km/s in the upper and 6.5–6.9 km/s in the lower crust. Towards the center of the basin, the Moho depth decreases from 27 to 22 km. Local variations of the velocity gradient can be attributed to previously postulated shear zones like the Pelusium Line, the Damietta–Latakia Line and the Baltim–Hecateus Line. Both layers of the crystalline crust are continuous and no indication for a transition from continental to oceanic crust is observed. These results are confirmed by gravity data. Comparison with other seismic refraction studies in prolongation of our profiles under Israel and Jordan and in the Mediterranean Sea near Greece and Sardinia reveal similarities between the crust in the Levantine Basin and thinned continental crust, which is found in that region. The presence of thinned continental crust under the Levantine Basin is therefore suggested. A β-factor of 2.3–3 is estimated. Based on these findings, we conclude that sea-floor spreading in the Eastern Mediterranean Sea only occurred north of the Eratosthenes Seamount, and the oceanic crust was later subducted at the Cyprus Arc.     

Gravity and multichannel seismic reflection constraints on the lithospheric structure of the Canary Swell

Deep penetrating multichannel seismic reflection and gravity data have been used to study the lithospheric structure of the Canary Swell. The seismic reflection data show the transition from undisturbed Jurassic oceanic crust, away from the Canary Islands, to an area of ocean crust strongly modified by the Canary volcanism (ACV). Outside the ACV the seismic records image a well layered sedimentary cover, underlined by a bright reflection from the top of the igneous basement and also relatively continuous reflections from the base of the crust. In the ACV the definition of the boundary between sedimentary cover and igneous basement and the crust-mantle boundary remains very loose. Two-dimensional gravity modelling in the area outside the influence of the Canary volcanism, where the reflection data constrain the structure of the ocean crust, suggests a thinning of the lithosphere. The base of the lithosphere rises from 100 km, about 400 km west of the ACV, to 80 km at the outer limit of the ACV. In addition, depth conversion of the seismic reflection data and unloading of the sediments indicate the presence of a regional depth anomaly of an extension similar to the lithospheric thinning inferred from gravity modelling. The depth anomaly associated with the swell, after correction for sediment weight, is about 500 m. We interpret the lithospheric thinning as an indication of reheating of old Mesozoic lithosphere beneath the Canary Basin and along with the depth anomaly as indicating a thermal rejuvenation of the lithosphere. We suggest that the most likely origin for the Canary Islands is a hot spot.     

The Marine VHR 2.5-D Seismic Brute Stack Cube as a Feasible Tool for Low Budget Investigation and Research

Between 1997 and 1999 several marine seismic surveys were carried out in Kiel Bay aimed towards the development of a three-dimensional acquisition and interpretation technique for small scale subsurface structures using high-frequency sources and multichannel streamers. The data set was recently revisited by the author and reprocessed to obtain a multichannel stacked seismic data cube. Nominal hydrophone positions are deduced by determining offsets from first arrival times and estimating the hydrophone positions under consideration of the ships track. Processing towards a ‘seismic cube’ mainly comprised CMP sorting, constant velocity NMO correction and stacking. The resulting VHR 3-D seismic ‘brute stack cube’ reveals rich structural details. The fluvial Pleistocene channel system already documented in an earlier publication was tracked further to the north. It is situated below a flat cover of gas-bearing Holocene sediments, which locally constitute the seafloor. This till-horizon is superimposed on a second till layer showing strong topographic variations. Seismic signal phase and shielding effects indicate the possible presence of gas in these formations. This case history demonstrates that the VHR 3-D seismic method is a feasible tool for low budget investigation and research.     

Seismic expression of contourites and related deposits: a preface

Contourites are widespread throughout the deep sea, ranging from those that build up individually distinct bodies (mounded drifts) to those that occur closely interbedded with other deep-water facies. Although seismic data should not be used to make a firm identification of contourites without supporting evidence, much progress has been made in determining the combination of seismic criteria that best represent contourite deposits. With some modifications and additions, these criteria are in broad agreement with those recently proposed by Faugères and colleagues (1999). The papers in this special issue are mainly drawn from those presented at an IGCP workshop held in Trieste, Italy (October 2000), together with selected additions. Some of the new and important topics covered include: discussion of bottom-current controlled deposits on continental shelves and in large lakes (shallow-water and lacustrine `contourites'); the unique characteristics of high-latitude contourite systems that show interaction with turbidity current, debris flow and glacigenic processes; and the importance of local slope topography, developed as a result of synsedimentary tectonics and downslope mass movement, in focussing and shaping drift deposits (infill and fault-related drifts). An introduction to the powerful use of 3D seismic data in understanding contourite systems points the way towards important future research.     

地震属性技术在圈定岩浆侵入煤层范围的应用

我国东部煤田普遍存在岩浆侵入现象,其对煤矿安全生产影响极大,因此准确划分岩浆侵入煤层范围是煤炭生产迫切需要解决的关键问题之一。通过分析岩浆岩、天然焦、煤等物性特征,构建地质模型进行正演模拟,证实其在振幅、相位、频率等存在明显差异。以山东某煤矿3煤层为例,利用单属性技术、多属性聚类分析及谱分解技术等结合常规方法,圈定了煤、天然焦分界线及岩浆侵入范围。通过矿方布设的10个钻孔验证,与解释结果比较吻合。     

Quaternary contourite drifts of the Western Spitsbergen margin

The study of contourite drifts is an increasingly used tool for understanding the climate history of the oceans. In this paper we analyse two contourite drifts along the continental margin west of Spitsbergen, just south of the Fram Strait where significant water mass exchanges impact the Arctic climate. We detail the internal geometry and the morphologic characteristics of the two drifts on the base of multichannel seismic reflection data, sub-bottom profiles and bathymetry. These mounded features, that we propose to name Isfjorden and Bellsund drifts, are located on the continental slope between 1200 and 1800 m depth, whereas the upper slope is characterized by reduced- or non-deposition. The more distinct Isfjorden Drift is about 25 km wide and 45 km long, and over 200 ms TWT thick. We revise the 13 years-long time series of velocity, temperature, and salinity obtained from a mooring array across the Fram Strait. Two distinct current cores are visible in the long-term average. The shallower current core has an average northward velocity of about 20 cm/s, while the deeper bottom current core at about 1450 m depth has an average northward velocity of about 9 cm/s. We consider Norwegian Sea Deep Water episodically ventilated by relatively dense and turbid shelf water from the Barents Sea responsible for the accumulation of the contourites. The onset of the drift growth west of Spitsbergen is inferred to be about 1.3 Ma and related to the Early Pleistocene glacial expansion recorded in the area. The lack of mounded contouritic deposits on the continental slope of the Storfjorden is related to consecutive erosion by glacigenic debris flows. The Isfjorden and Bellsund drifts are inferred to contain the record of the regional palaeoceanography and glacial history and may constitute an excellent target of future scientific drilling.     

浅层双分量地震在黄土公路地质缺陷探测中的应用研究

针对我国西部黄土公路下伏大量地质缺陷（灾害）及其对公路交通建设和运营的危害性,将多分量地震勘探技术引入浅层黄土公路工程勘探中.根据黄土公路地质缺陷独特的复杂性,重点对黄土地区浅层双分量地震勘探技术的数值模拟与观测系统优化设计、高品质地震资料采集试验做了深入研究,并在已知暗穴和危及公路的地裂缝发育区开展了双分量地震探测.实际探测结果表明：多分量地震勘探技术能分辨出规模大于1 m,埋深小于30 m的黄土公路地质缺陷.     

Basement control on oblique thrust sheet evolution: seismic imaging of the active deformation front of the Central Andes in Bolivia

In the area of the Bolivian Orocline, we examine the deformation pattern associated with the active development of a new thrust sheet. A dense grid of reprocessed 2-D seismic lines from hydrocarbon exploration industry is interpreted and a 3-D simplified structural and kinematic model is deduced. In the Boomerang Hills, onlapping Paleozoic and foredeep sediments are detached from the underlying S-dipping basement. They are thrust northeastwards by less than 2 km. Two zones can be differentiated along the Andean deformation front: (1) a W–E to NW–SE striking frontal segment of predominantly orthogonal shortening, comprising a thrust and anticline system; (2) a WSW–ENE striking lateral zone of oblique shortening within a complex system of thin-skinned strike–slip faults and minor folds. The deformation front always follows a pronounced edge in the topography of the top basement surface close to the boundary of the Paleozoic basin. The observed deformation pattern indicates intensified strain partitioning caused by the interaction of contraction direction and basement topography, which provides a near oblique ramp for the onlapping wedge of sediments. The SW–NE thrusting direction is divided into orthogonal and tangential components. These are accommodated by convergent and strike–slip structures, respectively, which sole into a common detachment horizon. The structural evolution of the new thrust sheet in the Bolivian Orocline is primarily controlled by the paleorelief of the Brazilian Shield because: (1) the shape of the basement affects the taper of the thrust wedge and localizes the deformation front and (2) small asperities in/close to the top of the basement promote fault localization. The coincidence of a relatively high basement position and a structural high of the Eastern Cordillera leads to the conclusion that the shape of the Brazilian Shield also controls the structural evolution of the pronounced eastern border of the Bolivian Orocline.     

Seismo-stratigraphic evidences for deep base level control on middle to late Pleistocene drift evolution and mass wasting along southern Levant continental slope (Eastern Mediterranean)

Since early Pliocene, a counterclockwise surface gyre transported Nile derived silt and clay northeastwards along the Levant coast, where a basinward prograding plastered drift emerged. Based on high-resolution seismic reflection data we develop a middle to late Pleistocene sequence stratigraphic scheme for this plastered drift. For creating stacked sections of the seismic data we used the common reflection surface (CRS) stack technology which enhanced lateral reflection continuity and visibility of deep reflections. The shelf comprises stratigraphic sequences which show classical, systems tract like stacking patterns of sea level controlled sequences such as offlapping forced regression deposits or diachronous ravinement surfaces which formed during base level rise. On the slope, base level was periodically located well below the wave base and thus rather controlled by hydrodynamics, presumably by high-velocity contour currents. Hence, the term ‘deep base level’ is introduced. The deep base level controlled especially down and backstepping slope deposits. This example shows that care has to be taken when interpreting subsurface data containing typical systems tract like seismic sequences, since such geometries do not necessarily imply shallow water deposition of the sediments. A chronostratigraphic analysis based on the seismic stratigraphy indicates that base level fluctuations were related to eccentricity driven glacio-eustatic fluctuations. Periodic mass wasting, facilitated by foreset over-steepening and possibly triggered by salt tectonics or erosion by the contour current occurred during late base level fall or early base level rise.