New data on the age of dolerites and basalts of Mendeleev Rise (Arctic Ocean)

We present results of ⁴⁰Ar/³⁹Ar isotopic investigations concerning the dating of dolerites and basalts that were sampled during the Arctica-2012 polar expedition. Basalts were sampled by means of deep underwater drilling with wells up to 2 m in outcrops on the seafloor (basalts), and dolerite samples were obtained from the bottom of an escarp of Mendeleev Rise using a manipulator on the research submarine. The analysis results of the obtained mono-mineral fractions (amphibole, plagioclase, pyroxene) from the studied rocks yielded an Early Paleozoic age of the dolerites and basalts from Mendeleev Rise. The oldest ages obtained for amphibole reach 471.5 ± 18.1 and 466.9 ± 3.3 Ma, which corresponds to the Early-Middle Ordovician. The isotopic composition of argon was measured on two mass spectrometers: the Micromass Noble Gas 5400 (UK) and the Thermo Scientific Argus (Germany). The determined Early Paleozoic age of igneous rocks of Mendeleev Rise and seismic data obtained during the last Russian expedition Arctica-2012 [2] let us suppose that this continental block of the Earth’s crust has a Precambrian basement similar to the basement identified for the New Siberian islands including the De Long archipelago.     

Typification of the Earth’s crust of Central Arctic Uplifts in the Arctic Ocean

The modern views on the structure of the oceanic and continental crust are discussed. The presented geological-geophysical information on the deep structure of the Earth’s crust of the Lomonosov Ridge, Mendeleev Rise, and Alpha Ridge, which make up the province of the Central Arctic Uplifts in the Arctic Ocean, is based on CMP, seismic-reflection, and seismic-refraction data obtained by Russian and Western researchers along geotraverses across the Amerasia Basin. It is established that the crust thickness beneath the Central Arctic Uplifts ranges from 22 to 40 km. Comparison of the obtained velocity sections with standard crust sections of different morphostructures in the World Ocean that are underlain by the typical oceanic crust demonstrates their difference with respect to the crustal structure and to the thickness of the entire crust and its individual layers. Within the continental crust, the supercritical waves reflected from the upper mantle surface play the dominant role. Their amplitude exceeds that of head and refracted waves by one to two orders of magnitude. In contrast, the refracted and, probably, interferential head waves are dominant within the oceanic crust. The Moho discontinuity is the only first-order boundary. In the consolidated oceanic crust, such boundaries are not known. The similarity in the velocity characteristics of the crust of the Alpha Ridge and Mendeleev Rise, on the one hand, and the continental crust beneath the Lomonosov Ridge, on the other, gives grounds to state that the crust of the Mendeleev Rise and Alpha Ridge belongs to the continental type. The interference mosaic pattern of the anomalous magnetic field of the Central Arctic Uplifts is an additional argument in favor of this statement. Such patterns are typical of the continental crust with intense intraplate volcanism. Interpretation of seismic crustal sections of the Central Arctic Uplifts and their comparison with allowance for characteristic features of the continental and oceanic crust indicate that the Earth’s crust of the uplifts has the continental structure.     

Sedimentary Rocks in the Basement of the Alpha–Mendeleev Rise,Arctic Ocean

Lithology and Mineral Resources - The results of petrographic, petrogeochemical and isotopic studies of sedimentary rocks obtained from bedrock outcrops of the Alpha–Mendeleev Rise, Arctic...     

Structure of the Earth’s crust in the northern part of the Barents–Kara region along the 4-AR DSS profile

The 1370 km long 4-AR reference profile crosses the North Barents Basin, the northern end of the Novaya Zemlya Rise, and the North Kara Basin. Integrated geophysical studies including common deep point (CDP) survey and deep seismic sounding (DSS) were carried out along the profiles. The DSS was performed using autonomous bottom seismic stations (ABSS) spaced 10–20 km apart and a powerful air gun producing seismic signals with a step size of 250 m. As a result, detailed P- and S-wave velocity structures of the crust and upper mantle were studied. The basic method was ray-tracing modeling. The Earth’s crust along the entire profile is typically continental with compressional wave velocities of 5.8–7.2 km/s in the consolidated part. Crustal thickness increases from 30 km near the islands of Franz Josef Land to 35 km beneath the North Barents Basin, 50 km beneath the Novaya Zemlya Rise, and 40 km beneath the North Kara Basin. The North Barents Basin 15 km deep is characterized by unusually low velocities in the consolidated crust: The upper crust layer with velocities of 5.8–6.4 km/s has a thickness of about 15 km beneath the basin (usually, this layer wedges beneath deep sedimentary basins). Another special property of the crust in the North Barents Basin is the destroyed structure of the Moho.     

Crustal structure measurements near FRAM II in the pole abyssal plain

An extensive refraction profiling program was carried out during the FRAM II experiment (March–May, 1980) in the eastern Arctic Ocean. Two structural areas were covered: north of the ice camp (86°N, 24°W) into the basin of the Pole Abyssal Plain and south onto the flanks of the Morris Jesup Rise. Digital multichannel data on an 800 by 800 m, 24 channel hydrophone array and a single 2-component ocean bottom seismometer (OBS) were recorded for offsets from 2.5 to 100 km. Arrival times, amplitudes and phase velocities of the seismic signals recieved on the hydrophone array were determined using high resolution array processing. From these measurements and the OBS data, preliminary velocity structural models of the crust have been derived. For the purposes of this paper, 2 refraction lines have been analyzed, a 40 km line on a flat region of the Pole Abyssal Plain and an 86 km line on a slightly dipping region taken as the drifting ice camp shoaled on the Morris Jesup Rise. These preliminary analyses yield a sedimentary layer with a gradually increasing velocity 1.5–2 km thick. This cover overlays a crust with a thin layer 2 (< 1 km) and yields a total ocean bottom to mantle thickness of 4–7 km.     

Geophysical transects of the Labrador Sea: Labrador to southwest Greenland

In 1977 the Federal Institute for Geosciences and Natural Resources, Hannover, carried out a large scale multichannel reflection seismic survey in the Labrador Sea. This survey provided an opportunity for the direct comparison of the geologic structure of the Labrador and Greenland margins. The seismic records across the Labrador Shelf show a thick, prograding sedimentary wedge consisting of several seismic sequences onlapping an acoustic basement that dips steeply seaward. The surface of the acoustic basement is irregular below the continental slope, indicating Late Cretaceous—Early Tertiary faulting. The thick sedimentary section below the slope is divided by an unconformity, tentatively identified as Late Tertiary in age, into two seismic megasequencies which can be subdivided. The acoustic basement on the Greenland side is also strongly faulted but is overlain, in the south, by a thin sedimentary section. The sediment cover thickens on the Greenland Shelf to the north as the shelf becomes wider.As with more southerly parts of the western Atlantic margin, a positive free-air anomaly (30–50 mgal) lies landward of the shelf break off Labrador and a smaller negative anomaly follows the base of the slope. Similar, but generally narrower features are observed along the Greenland margin. West of the negative anomaly off the Greenland slope a narrow band of lower amplitude positive anomalies tends to be associated with an acoustic basement high observed in the reflection profiles. A landward negative gradient in the simple Airy isostatic anomaly across this margin suggests that the ocean—continent boundary is related to this high.Detailed magnetic measurements across the northern Labrador margin show that well-developed oceanic anomalies trending north-northwest lie east of the large Labrador Shelf gravity high, beyond the 2000 m isobath. Landward of these magnetic anomalies is a quiet magnetic zone within which the linear gravity high is parallel to the shelf break and correlates with a deep, sediment-filled basin. It is inferred that oceanic-type crust or greatly-attenuated continental crust underlies this basin and that continental crust thickens markedly westward of the gravity high over a distance of about 50 km.     

THICKNESS OF THE CRUST AND ITS RELATIONSHIP TO RELIEF AND GRAVITY ANOMALIES

The effect of different crustal thickness on a regional gravity field may be differentiated, as a first approximation, into-three layers: 1) sedimentary, 2) granitic, and 3) basaltic. The study of complex “wave pictures” obtained in deep seismic sounding has lead to differentiation of the crust as continental, oceanic, and transitional, with a general relationship existing between the surface tectonics of the crust and its deeper structures. The crust is thickest in the mountain regions (40 km-80 km) as against an average for the platforms of about 25 km-35 km. It appears that there are two particularly conspicuous gravity and seismic discontinuities in the crust; one between the sedimentary mantle and the so-called crystalline layer and the other between the latter and the M surface. Tentative estimations of crustal thickness are as follows: the Russian Platform and the north of the western Siberian Platform; 30 km-34 km; the Black Sea about 24 km; the entire south, southeast and east of the U. S. S. R. are marked by greater depth with the Pamirs having a thickness of over 70 km; in the Caucasus the M surface lies below 45 km; in the Northern Kazakhstan the crust is 34 km-36 km thick; in the Altay thickness of around 50 km are indicated; in the Eurasian continent, Tibet has the thickest crust, the gravity minimum indicating about 85 km; in the Verkoyansk region the M surface is over 43 km. Large areas of the Arctic Ocean is occupied by the shelf with a thickness similar to that in the north of the country. This suggests that a considerable stretch of the ocean adjacent to the northern shores of the U. S. S. R. has a continental type. The crust thins rapidly to the north to about 10 km. Along the Pacific coast the M surface is about 33 km, the shelf zone is rather narrow including the Sea of Okhotsk. Toward the ocean and the Kuriles the crust thins rapidly to 10 km. -- C. E. Sears.     

Geochemistry of organic matter of bottom sediments in the rises of the central Arctic Ocean

Based on geomorphological, lithological, and facial characteristics of the East Arctic continental margin, we studied the main factors controlling the Late Cenozoic supply of organic matter (OM) to the bottom sediments of the Central Arctic rises of the Arctic Ocean. Complex analysis of dispersed OM in the samples taken during the expeditions of the R/V “Akademik Fedorov” in 2000 and 2005 showed a significant difference between the sediments of the Lomonosov Ridge and Mendeleev Rise. The bottom sediments of the latter are strongly transformed and lack terrigenous components, as evidenced results from the main geochemical characteristics (contents of C_org, C_carb, N_org, bitumens, and humic acids) and the composition and distribution of hydrocarbon molecular markers (alkanes, saturated and aromatic cyclanes). The obtained data evidence that ancient sedimentary rocks containing genetically uniform deeply transformed (up to mesocatagenesis) OM played a significant role in the formation of the Pleistocene–Holocene sediments of the axial part of the Mendeleev Rise.     

A new method for identification of protocatagenesis, mesocatagenesis, and apocatagenesis zones in a terrigenous sedimentary cover by applying layer seismic velocities

This method pertains to oil and gas geology and to geology of sedimentary basins prospective for oil and gas. It includes identifying catagenesis zones in drilled areas within the sedimentary cover of the basin based on assay results for drill cores and cuttings using the standard methods. Analysis is primarily made in order to determine rock catagenesis based on the optical characteristics of vitrinite. A correlation between catagenesis zones and layer seismic velocities obtained from regional and exploration seismic data is made for a drilled area. Both the layer seismic velocities and the degree of rock catagenesis increase with depth under the influence of increasing rock density. Correlations between layer velocities and the degree of rock catagenesis have been established. The following ratios have been determined for the Scotian shelf, Canada, and the Barents shelf, Russia: a protocatagenesis zone (the cap) corresponds to layer seismic velocities (V _lay) of 1.5–3.3 km/s, a mesocatagenesis zone (the principal hydrocarbon generation area) corresponds to V _lay of 3.3–5.0 km/s, and an apocatagenesis zone (an area with a very low hydrocarbon potential) corresponds to V _lay of over 5.0 km/s. An advantage of the new method of identification of catagenesis zones is that it can be used prior to drilling. Its conceptual originality and cost efficiency lie precisely in this.     

An explosion seismology investigation of the continental margin west of the Hebrides, Scotland, at 58°N

During summer 1975, a line of large shots was fired across the continental margin between the Rockall Trough and the Hebridean shelf along 58°N. Arrivals were observed at temporary seismic stations set up across Scotland and in northwestern Ireland. A clear P₂ phase was observed to cross the margin and a converted phase P₁ also seen on the records is interpreted as travelling in the sub-sedimentary oceanic crust of Rockall Trough and in the upper continental crust beneath the shelf.The continental crust beneath the Hebridean shelf is estimated to be 27 ± 2 km thick, with P_g = 6.22 ± 0.03 km/s and P_n = 8.01 ± 0.04 km/s as determined by time-term analysis. P_g delays on the outer shelf are interpreted in terms of a seaward thickening wedge of Mesozoic sediments which pre-date the split. P_n beneath the Rockall Trough was poorly determined at 8.20 ± 0.17 km/s and the Moho is estimated to be 18 ± 2 km deep at 58°N. This and other seismic and gravity work indicates a northward thickening of the crust along the Rockall Trough, accounting for the northward decrease in the height of the slope.Our results, and those of gravity interpretations, indicate a relatively abrupt transition between continental and oceanic crust, possibly correlating with the lack of major shelf subsidence. This is attributed to a relatively cool origin for this margin. The main thinning of the continental crust beneath the slope is attributed to outslip of continental crustal material into and beneath the newly forming oceanic crust during the first few million years after the split, possibly enhanced by pre-split stretching.     

Crustal structure beneath the Faroe Islands and the Faroe–Iceland Ridge

We have mapped the transition from the continental Faroe block (the Faroe Islands and surrounding shelf) to the thickened oceanic crust of the Faroe–Iceland Ridge in the North Atlantic using the results of a detailed sea-to-land seismic profile with wide-angle to normal-incidence recordings of explosive and airgun shots fired at sea along the Faroe–Iceland Ridge. Interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30±3-km-thick oceanic crust, with a gradual transition to ancient continental crust from 100 to 40 km northwest of the Faroe Islands, close to the shelf edge. This confirms that the crust beneath the Faroe Islands, which may be up to 46 km thick, comprises continental material in agreement with previous seismic and geochemical results. Results suggest that the upper 5.2±0.7 km of the Faroe crust consists of Tertiary basalts generated during continental breakup, overlying the continental crust beneath. The lower crust, where seismic constraint is poor, may exhibit high seismic velocities (7.1–7.6 km s⁻¹) which we attribute to underplating or intrusion by mafic melts during continental breakup in the early Tertiary.     

Seismic models of inner parts of the Euro-Asian continent and its margins

Analogies are drawn between continental and continental margin structures on the basis of seismic data on the crustal structure of Eurasia and its Atlantic margins. Crustal thinning from the inner parts of the continent to its margins is observed to be a general feature common to the formation of deep midland depressions and sedimentary basins of shelf zones. The latter are characterized by crustal thinning and its assimilation. These phenomena cannot be explained solely be sea-floor spreading effects in the process of active rifting and formation of oceanic crust. It appears that the main role in the formation of the margins in played by processes of mantle erosion in connection with heating at continental margins and with the migration of mantle material to the lower part of the crust.     

New data on tectonics of Mendeleev Ridge and adjacent geological structures

The comprehensive analysis of potential field data and recent seismic data revealed two systems of fractures bounding horsts and grabens in terms of the Mendeleev Ridge. The northern part of the ridge is marked by development of pull-apart structures indicating the former existence of oblique extension settings. The area between Mendeleev and Alpha ridges is occupied by a wide NW?SE–extending sinistral strike-slip zone. It is concluded that these ridges are of continental origin representing former parts of Arctida (Hyperborea) in the pre-Cretaceous time. The ridges were separated and their crust significantly altered during Cretaceous tectono-magmatic activation in the region.     

Geology and tectonics of the northeast Russian Arctic region,based on seismic data

The structure of the sedimentary cover and acoustic basement in the northeastern Russian Arctic region is analyzed. Beneath the western continuation of the North Chukchi trough and Vil’kitskii trough, a Late Caledonian (Ellesmere) folded and metamorphozed basement is discovered. It is supposed that Caledonides continue further into the Podvodnikov Basin until the Geofizikov branch. A large magnetic anomaly in the Central Arctic zone has been verified by seismostratigraphic data: the acoustic basement beneath the Mendeleev (and partially Alpha) Ridge is overlain by trapps. Wave field analysis showed that the acoustic basement of the Lomonosov Ridge has folded structure, whereas beneath the Mendeleev Ridge, the sporadic presence of a weakly folded stratum of Paleozoic platform deposits is interpreted. It is supposed that the Caledonian and Late Cimmerian fold belts in the periphery of the Arctida paleocontinent appeared as a result of collision between arctic continental masses and southern ones. After Miocene extension and block displacements identified from appearance of horsts, grabens, and transverse rises both on the shelf and in the ocean, a general subsidence took place and the present-day shelf, slope, and the deepwater part of the Arctic Ocean formed.     

建南地区上二叠统长兴组生物礁地震沉积学

建南地区二叠系长兴组时期沉积相平面展布一直存在较大争议,严重制约生物礁滩发育区带预测。在实际研究中,以碳酸盐岩沉积理论为指导,运用地震沉积学研究思路与方法,利用单井资料对地震相反射特征进行地质意义解析,建立沉积相地震识别模式,并且运用古地貌恢复、属性分析等技术进行沉积相平面分布与演化分析。研究结果表明：生物礁井震结合进行层序标定后,地震相位特征能够较好地反映生物礁滩发育期次;结合地震剖面相与平面相识别,在研究区中部主要为低频、弱振幅反射的陆棚相,能够说明开江-梁平陆棚延伸到研究区;地震多属性分析能够较为真实地对沉积相边界与生物礁滩分布范围进行刻画;等时地层切片反映出三级层序SQ1后期为生物礁主要发育时期,SQ2时期海平面变浅,暴露滩体发育部位产生迁移。     

贵州东部中上寒武统层序地层学及台地演化

通过对贵州东部丹寨一三都中上寒武统两个碳酸盐岩露头剖面的层序地层学研究，识别出3个(三级)层序及8个体系域，并认为由于构造升降运动，两露头剖面的沉积环境发生变化，该区台地类型由中寒武统的缓坡型陆棚演变成上寒武统的镶嵌陆架型台地。研究表明，自中元古宙晚期以来，研究区经历了由洋壳过渡到陆壳的演变过程，体现在该区自中寒武世至晚寒武世由广阔的坡缓底平的广海陆棚沉积，转为台地(斜坡)-广海(盆地)沉积环境。     

Ray path interpretation of the crustal structure beneath Saudi Arabia

Interpretation of a long-range seismic refraction line in Saudi Arabia has shown that beneath the Arabian Shield velocity generally increases with depth, from about 6 km s⁻¹ at the surface to about 7 km s⁻¹ at the top of the crust-mantle transition zone. The base of this transition zone (Moho) occurs at 37–44 km in depth. Intracrustal discontinuities can also be recognized, the most important being in the 10–20 km-depth range and separating the upper from the lower crust. Laterally, the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions. Beneath the Red Sea shelf and coastal plain the crust, including 4 km of sediments, is only 15–17.5 km thick. With the aid of both seismic and gravity data an abrupt, steeply dipping transition from the crust of the Red Sea shelf and coastal plain to that of the Arabian Shield has been derived. With a jump of more than 20 km in Moho depth, this appears to be the major discontinuity between the Red Sea depression and the Arabian continental shield.     

A three dimensional perspective on the evolution of Archaean crust: LITHOPROBE seismic reflection images in the southwestern Superior province

15011993

Abstract

In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone.

Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico.

Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.     

冲绳海槽西部陆坡第四纪沉积地层划分

冲绳海槽西部陆坡是认识深海斜坡沉积的重要区域。通过对高分辨率地震资料的精细解释,结合已有的研究成果,在冲绳海槽西部陆坡海底以下识别出4个主要的地震层序界面,相应地划分出四个主要地震层序,各层序分别对应全新世沉积层(Q4)、晚更新世沉积层(Q3)、中更新世沉积层(Q2)和早更新世沉积层(Q1)。从陆坡上部往下由斜交前积反射结构过渡到杂乱的反射结构,在地震剖面上可识别出杂乱的丘形反射单元,是滑塌体和重力流沉积的典型地震反射特征,反映出一种高能的、极不稳定的沉积环境。海槽轴部以平行-亚平行的地震反射特征为主,显示了稳定的深海-半深海的沉积环境。地震反射结构的多样性反映了冲绳海槽西部陆坡沉积环境的复杂性和沉机作用类型的多样性,沉积地层结构是多种因素共同作用的结果。东海陆架和冲绳海槽发育相同的第四纪地层垂向序列,同时冲绳海槽西部陆坡与东海陆架第四纪沉积层在层序界面、沉积层厚度、变形程度和产状等方面存在着差异,单靠地震资料来进行两个地区的地层对比存在着不确定性。     

Morphology and geology of the continental shelf and upper slope of southern Central Chile (33°S–43°S)

The continental shelf and slope of southern Central Chile have been subject to a number of international as well as Chilean research campaigns over the last 30 years. This work summarizes the geologic setting of the southern Central Chilean Continental shelf (33°S–43°S) using recently published geophysical, seismological, sedimentological and bio-geochemical data. Additionally, unpublished data such as reflection seismic profiles, swath bathymetry and observations on biota that allow further insights into the evolution of this continental platform are integrated. The outcome is an overview of the current knowledge about the geology of the southern Central Chilean shelf and upper slope. We observe both patches of reduced as well as high recent sedimentation on the shelf and upper slope, due to local redistribution of fluvial input, mainly governed by bottom currents and submarine canyons and highly productive upwelling zones. Shelf basins show highly variable thickness of Oligocene-Quaternary sedimentary units that are dissected by the marine continuations of upper plate faults known from land. Seismic velocity studies indicate that a paleo-accretionary complex that is sandwiched between the present, relatively small active accretionary prism and the continental crust forms the bulk of the continental margin of southern Central Chile.