Insights into the lithospheric structure and tectonic setting of the Barents Sea region from isostatic considerations

We study the tectonic setting and lithospheric structure of the greater Barents Sea region by investigating its isostatic state and its gravity field. 3-D forward density modelling utilizing available information from seismic data and boreholes shows an apparent shift between the level of observed and modelled gravity anomalies. This difference cannot be solely explained by changes in crustal density. Furthermore, isostatic calculations show that the present crustal thickness of 35–37 km in the Eastern Barents Sea is greater than required to isostatically balance the deep basins of the area (>19 km). To isostatically compensate the missing masses from the thick crust and deep basins and to adequately explain the gravity field, high-density material (3300–3350 kg m⁻³) in the lithospheric mantle below the Eastern Barents Sea is needed. The distribution of mantle densities shows a regional division between the Western and Eastern Barents and Kara Seas. In addition, a band of high-densities is observed in the lower crust along the transition zone from the Eastern to Western Barents Sea. The distribution of high-density material in the crust and mantle suggests a connection to the Neoproterozoic Timanide orogen and argues against the presence of a Caledonian suture in the Eastern Barents Sea. Furthermore, the results indicate that the basins of the Western Barents Sea are mainly affected by rifting, while the Eastern Barents Sea basins are located on a stable continental platform.     

Transitional continental–oceanic structure beneath the Norwegian Sea from inversion of surface wave group velocity data

We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50  s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18  km in the Norwegian Sea area and 9  km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9  km  s⁻¹ for most paths. An average lithospheric thickness of 60  km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30  km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.     

A deep seismic profile of 800 km length recorded in southern Queensland, Australia

Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins.     

环北极沉积盆地结构与构造演化特征——来自环北极地质长剖面的证据

通过对北极地区不同盆地结构的系统研究与对比分析,绘制了环北极沉积盆地群长剖面,剖面全长约13 000 km,涉及季曼-伯朝拉盆地等15个盆地和微陆。由于整个剖面尺度巨大,在若干缺乏详细资料的地区采用将邻近地区的其他剖面平移到本剖面中,或由其两侧的剖面地层和构造形式推测。剖面涉及的盆地多属叠合性质,涵盖克拉通盆地、裂谷盆地、前陆盆地、被动大陆边缘盆地等多种盆地类型;各区域沉积厚度差异显著,最厚的东巴伦支海等盆地沉积了自古生界至新生界的地层,沉积厚度可达近15 km ,而沉积最薄处的拉普捷夫海等盆地则只发育了从中生界上白垩统至新生界的沉积,厚度不超过4 km。以北大西洋—北冰洋洋中脊为界,北极地区分属欧亚板块和北美板块,很多盆地为大陆的一部分或大陆向北冰洋的延伸部分（大陆架）。各盆地的发育主要受波罗的板块、西伯利亚板块与劳伦古陆显生宙以来的构造演化控制;加里东运动、埃尔斯米尔运动、海西运动（乌拉尔运动）及大西洋洋脊自南向北的扩张对环北极盆地均有显著影响,具体表现为造成盆地类型的改变、改变盆地沉降速率等。     

Anelasticity of the Crust and Upper Mantle of South America From the Inversion of Observed Surface Wave Attenuation

Fundamental-mode Rayleigh and Love waves generated by several earthquakes situated along great-circle paths between pairs of seismograph stations have been analysed to obtain coefficients of attenuation, group velocities, phase velocities, and specific quality factors in the period range 18–80s in two regions of the South American continent. One set of paths crosses the shield region which lies on the eastern coast and another set traverses the mountainous region inland. the average attenuation coefficient values are clearly higher in the tectonically active western region throughout the entire period range than in the eastern or shield region.
Inversion of the attenuation data yielded shear wave internal friction ( Q ^-1_β) models as a function of depth in the crust and upper mantle in both regions. A low- Q zone below the lithosphere is prominent in both regions. the results show that substantial variations of Q _β occur in the two regions of South America. the Q_β values were found to be inversely related to the heat flow values or to the temperature.     

Anomalous propagation of surface waves in the Barents Sea as inferred from NORSAR recordings

Summary. NORSAR recordings of Rayleigh waves generated by presumed nuclear explosions on central and southern Novaya Zemlya and in northwestern Siberia have been studied. Using a frequency time analysing technique and correcting for presumed known dispersion effects across the Baltic Shield, dispersion curves for two different paths across the southern part of the Barents Sea were obtained. The curves are very unusual in that they give extremely low velocities even for periods up to 20 s. For the path to the middle part of the island, the inversion of the data gives a model with sediments and consolidated sediments down to 25 km, followed by a 15-km thick basaltic layer and an upper mantle with a P velocity as low as 7.9 km/s. For the path to the southern part of Novaya Zemlya the data inversion gives a somewhat different model with sediments and consolidated sediments down to 8 km, followed by a 17-km thick zone with velocities close to granitic and a 15-km thick layer with basaltic velocities. Again the upper-mantle P velocity is only 7.9 km/s. Other indications of lateral inhomogeneities in the Barents Sea are obtained by utilizing the array's capability to determine the angle of approach of seismic waves. It is demonstrated that reflections both from inhomogeneities in the Barents Sea and the continental margin off Norway can be detected. For waves from the southern end of the island, a reflection from a strong discontinuity close to the direct path to the middle part of the island is found, whereas signals from this area include a reflected wave possibly coming from the edge of the Svalbard platform.     

Gravity studies of the Rockall and Exmouth Plateaux using SEASAT altimetry

Abstract SEASAT altimetric measurements are used to determine the gravity anomalies across two passive continental margins: the western margin of the Rockall Plateau, UK, and the Exmouth Plateau off north-west Australia. The small gravity anomalies observed over the starved western margin of the Rockall Plateau require the existence of a major density contrast within the crust, as well as the Moho, and show that the elastic thickness is less than 5 km at the time of rifting. The gravity anomaly over the Exmouth Plateau is compared with the gravity anomaly calculated from the sediment loading of a thin elastic plate, taking account of the variation in crustal thickness. This comparison shows that the Exmouth Plateau also has a small effective elastic thickness of 5 km, even for loads emplaced between 60 and 120 Myr after rifting. Elastic thicknesses of about 5 km have also been reported for other sedimentary basins, and are to be expected if the rheological properties of the crust and mantle depend on the ratio of the present temperature to the melting temperature. Flexural effects are therefore likely to be of minor importance in sedimentary basins.     

Upper crustal shear velocity models from higher mode Rayleigh wave dispersion in Scotland

Summary. Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8–4.8 Hz, generated from a local earthquake of magnitude 3.7 M _L in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s⁻¹ in the upper 2 km, with a slow increase to around 3.8 km s⁻¹. P -wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments.     

Hydrography in the north–western Barents Sea, July–August 1996

CTD profiles from the north–western Barents August 1996, have been analysed and characteristics of have been compared with former analyses and investigations The barotropic and baroclinic modes of the Rossby radius of deformation have been estimated in order to give an estimate in order to give an estimate of the spatial scale of variations. The first baroclinic mode of the Rossby radius of deformation is estimated to be around 3 km. Cold halocline water (CHW) is found in the southern part of the investigation area, supporting a hypothesis that the production of CHW is located in the area around Storbanken, and not closer to the shelf break further north. Another hypothesis is proposed: tidal induced horizontal circulation and vertical currents may explain a northward transport of warmer water across sills and banks in the north–western Barents Sea.     

Deep seismic reflection and refraction profiling along the Aquitaine shelf (Bay of Biscay)

Summary. The 300 km ECORS - Bay of Biscay profile was carried out along the Aquitaine shelf and comprised a complete set of experiments including zero-offset and 7.5 km constant offset vertical seismic reflection and six expanding spread profiles. Large offset recordings were fundamental for the definition of the layered lower crust and the Moho, while ESPs provided decisive complementary information for the geological interpretation. These data show a strong variation in crustal thickness from about 20 km beneath the rifted Parentis basin, a failed arm of the oceanic Bay of Biscay, up to 35 km to the north below the Armorican shelf, in the Hercynian domain, and to the south below the Cantabria shelf, in the vicinity of the Pyrenean deformation front. The results have important implications for the behaviour of the crust during the formation of rifted sedimentary basins and during continental collision.     

Crust and upper mantle shear velocities from controlled sources

b
A two ship refraction profile was undertaken on the Australian continental shelf during the Banda Sea geophysical program, carried out by the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography and the Geological Survey of Indonesia. S waves originating close to the sea bottom were observed to distances of up to 1150 km at an array of stations in northern Australia.
These observations are interpreted as implying S mantle velocities of 4.60 km s^-1 from a depth of 45 km to a depth of 76 km and 4.72 km s^-1 below a depth of 76 km.
Ratios of the P and S travel times (V_p/V_s) have been determined to be 1.74 in the crust rising to a value of greater than 1.79 below a velocity discontinuity at a depth of 200 km. It is inferred that this high value arises because the effect of temperature is greater for S than for P .
Using the data from this and other studies in the shield region of Northern Australia it has been found that the _S travel times are significantly less than predicted by the Jeffreys—Bullen tables.     

Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data

The crustal and upper mantle structure of the northwestern North Island of New Zealand is derived from the results of a seismic refraction experiment; shots were fired at the ends and middle of a 575 km-long line extending from Lake Taupo to Cape Reinga. The principal finding from the experiment is that the crust is 25 ± 2 km thick, and is underlain by what is interpreted to be an upper mantle of seismic velocity 7.6 ± 0.1 km s⁻¹, that increases to 7.9 km s⁻¹ at a depth of about 45 km. Crustal seismic velocities vary between 5.3 and 6.36 km s⁻¹ with an average value of 6.04 km s⁻¹. There are close geophysical and geological similarities between the north-western North Island of New Zealand and the Basin and Range province of the western United States. In particular, the conditions of low upper-mantle seismic velocities, thin crust with respect to surface elevation, and high heat-flow (70–100 mW m⁻²) observed in these two areas can be ascribed to their respective positions behind an active convergent margin for about the past 20 Myr.     

Ottar Basin, SW Barents Sea: a major Upper Palaeozoic rift basin containing large volumes of deeply buried salt

Seismic mapping and gravity modelling of the Ottar Basin - a little studied Upper Palaeozoic graben in the south-western Barents Sea - demonstrates the presence of a major rift basin with large accumulations of unmobilized salt. Buried beneath thick, flat-lying Mesozoic strata, the NE-trending fault-bounded basin is at least 170 km long, varies in width between 50 and 80 km and coincides with a negative gravity anomaly of more than — 10 mgal. Seismic observations show that the south-western part is a half-graben tilted to the north-west whereas the north-eastern part appears to be more symmetric in shape. A large mass deficiency in the north-eastern part of the basin, indicated by a gravity anomaly of more than — 30 mgal, makes it necessary to postulate large amounts of salt within the basin. The preferred gravity model shows a total basin depth of 9.5 km, basin relief of 4.2 km and a salt volume of 6800 km³ corresponding to a 2.4-km-thick salt layer. Similar basin depths, but only 500–600 km³ of salt, are indicated beneath the Samson Dome in the south-western part of the basin. Unlike salt bodies in other Barents Sea basins, the thick salt deposit in the north-eastern part of the Ottar Basin is relatively unaffected by halokinesis. Interfingering of different basin facies, lack of tectonic reactivation of the basin and a relatively late differential loading by protruding cover strata probably explain these differences in development. The large size and voluminous salt deposits establish the Ottar Basin as one of the major Barents Sea evaporite basins and an important structural component of the Upper Palaeozoic rift system.     

Tectonics of the late Mesozoic wide extensional basin system in the China–Mongolia border region

The China–Mongolia border region contains many late Mesozoic extensional basins that together constitute a regionally extensive basin system. Individual basins within the system are internally composed of a family of sub‐basins filled with relatively thin sedimentary piles mostly less than 5 km in thickness. There are two types of sub‐basins within the basins, failed and combined, respectively. The failed sub‐basins are those that failed to continue developing with time. In contrast, the combined ones are those that succeeded in growing by coalescing adjacent previously isolated sub‐basins. Thus, a combined sub‐basin is bounded by a linked through‐going normal fault that usually displays a corrugated trace on map view and a shallower dip on cross‐section. Along‐strike existence of discrete depocenters and alternation of sedimentary wedges of different types validate the linkage origin of combined sub‐basins. Localized high‐strain extension resulted in large‐amount displacement on linked faults, but contemporaneously brought about the cessation of some isolated fault segments and the formation of corresponding failed sub‐basins in intervening areas between active linked faults. Some combined sub‐basins might have evolved into supradetachment basins through time, concurrent with rapid denudation of footwall rocks and formation of metamorphic core complexes in places. A tectonic scenario of the broad basin system can be envisioned as an evolution from early‐stage distributed isolated sub‐basins to late‐stage focused combined or/and supradetachment sub‐basins bounded by linked faults, accompanied by synchronous cessation of some early‐formed sub‐basins. Initiation of the late Mesozoic extension is believed to result from gravitational collapse of the crust that had been overthickened shortly prior to the extension. Compression, arising from collision of Siberia and the amalgamated North China–Mongolia block along the Mongol–Okhotsk suture in the time interval from the Middle to Late Jurassic, led to significant shortening and thickening over a broad area and subsequent extensional collapse. Pre‐ and syn‐extensional voluminous magmatism must have considerably reduced the viscosity of the overthickened crust, thereby not only facilitating the gravitational collapse but enabling the lower‐middle crust to flow as well. Flow of a thicker crustal layer is assumed to have occurred coevally with upper‐crustal stretching so as to diminish the potential contrast of crustal thickness by repositioning materials from less extended to highly extending regions. Lateral middle‐ and lower‐crustal flow and its resultant upward push upon the upper crust provide a satisfying explanation for a number of unusual phenomena, such as supracrustal activity of the extension, absence or negligibleness of postrift subsidence of the basin system, less reduction of crustal thickness after extension, and non‐compression‐induced basin inversion, all of which have been paradoxical in the previous study of the late Mesozoic basin tectonics in the China–Mongolia border region.     

Subsidence in the super-deep Pattani and Malay basins of Southeast Asia: a coupled model incorporating lower-crustal flow in response to post-rift sediment loading

Two Early Cenozoic rifts in Southeast Asia (beneath the Pattani and Malay basins) experienced only limited upper-crustal extension (β≤1.5); yet very thick post-rift sequences are present, with 6–12 km of Late Cenozoic terrestrial and shallow-marine sediment derived from adjacent sources. Conventional post-rift backstripping requires depth-dependent lithospheric thinning by β=2–4 to explain these tremendous thicknesses. We assess an alternative explanation for this post-rift subsidence, involving lower-crustal flow from beneath these basins in response to lateral pressure-gradients induced by the sediment loads and the negative loads arising from the erosion of their sediment sources. We calculate that increased rates of erosion in western Thailand in the Early Miocene placed the crust in a non-steady thermal state, such that the depth (and thus, the pressure) at the base of the brittle upper crust subsequently varied over time. Following such a perturbation, thermal and mass-flux steady-state conditions took millions of years to re-establish. In the meantime, the lateral pressure-gradient caused net outflow of lower crust, thinning the crust beneath the depocentre by several kilometres (mimicking the isostatic effect of greater crustal extension having occurred beforehand) and thickening it beneath the sediment source region. The local combination of hot crust and high rates of surface processes, causing lower-crustal flow to be particularly vigorous and thus making its effects more readily identifiable, means that the Pattani and Malay basins represent a set of conditions different from basins in many other regions. However, lower-crustal flow induced by surface processes will also occur to some extent, but less recognisably, in many other continental crustal provinces, but its effects may be mistaken for those of other processes, such as larger-magnitude stretching and/or depth-dependent stretching.     

The transition from subduction to continental collision: crustal structure in the North Canterbury region, New Zealand

The North Canterbury region marks the transition from Pacific plate subduction to continental collision in the South Island of New Zealand. Details of the seismicity, structure and tectonics of this region have been revealed by an 11-week microearthquake survey using 24 portable digital seismographs. Arrival time data from a well-recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous inversion for both hypocentres and velocity structure. The velocity structure is consistent with the crust in North Canterbury being an extension of the converging Chatham Rise. The crust is about 27 km thick, and consists of an 11 km thick seismic upper crust and 7 km thick seismic lower crust, with the middle part of the crust being relatively aseismic. Seismic velocities are consistent with the upper and middle crust being composed of greywacke and schist respectively, while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited.
The distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43S. To the south-west it is subhorizontal, while to the north-east it dips north-west at about 10. Fault-plane solutions for these earthquakes also change near 43S. For events to the south, P -axes trend approximately normal to the plate boundary (reflecting continental collision), while for events to the north, T -axes are aligned down the dip of the subducted plate (reflecting slab pull). While lithospheric subduction is continuous across the transition, it is not clear whether the lower crust near 43S is flexed or torn.     

Surface wave dispersion and Earth structure in south-eastern China

Summary. A reconnaissance study of crust and mantle structure in southeastern China was made using surface waves confined to that region from recent earthquakes. Data from the WWSSN stations ANP arid SEO, and from the digital stations TATO and MAT, were used to measure fundamental-mode group velocities of Love and Rayleigh waves over nine paths in south-eastern China, an area which has been technically quiet since the early Cenozoic. Crustal structure in this region is typical of stable continents, but shear-wave velocities in the uppermost mantle are low for a continent, 4.45 km s⁻¹ or less. Other seismological data support this observation.     

Deep seismic reflection profiles across the western Barents Sea

Summary. The continental crust beneath the western Barents Sea has been acoustically imaged down to Moho depths in a large scale deep seismic reflection experiment. A first-order pattern of crustal reflectivity has been established and the thickness of the crust determined. A number of features with important implications for the tectonics of the area have been discovered. The results are presented in the form of two transects.     

Southwest Barents Sea rift basin evolution: comparing results from backstripping and time‐forward modelling

We present tectonic models of progressive basin formation in the south‐west Barents Sea derived as part of the PETROBAR project (Petroleum‐related studies of the Barents Sea region). The basin architecture developed as a multi‐stage rift preceding the creation of the sheared/transtensional margin conjugate to NE Greenland. N‐ to NNE‐striking basins, with sediment thicknesses in places exceeding 15 km, are separated by basement highs. We use two basin analysis approaches, BMT? backstripping and TecMod?time‐forward modelling, to determine stretching factors through time along the profile PETROBAR‐07. This 550 km‐long profile derived from wide‐angle reflection/refraction seismic data acquired in 2007, coincident with deep multichannel seismic reflection data. Detailed stratigraphic analysis of the reflection profile, in concert with a dense grid of 2D profiles tied to wells, provides timing and water depth constraint for the models. Velocity analysis of the wide‐angle data provides constraint on the cumulative crustal stretching. The north‐west trending cross‐section extends from continental craton, at the Varanger Peninsula, to within 16 km of the interpreted continent–ocean boundary. Rifting along the profile was episodic, with four distinct phases of basin formation during the Carboniferous, the Late Permian–Triassic, the Late Jurassic–Early Cretaceous and the Late Cretaceous–Eocene. Collectively, the basins exhibit a general trend of younging, narrowing, and deepening oceanward, suggesting a gradual focusing of rifting prior to final breakup. Cumulative stretching factors derived from BMT and TecMod correlate well with observed crustal thinning, and the two models provide uncertainty bounds for stretching factors for the separate rift phases. In contrast to orthogonally rifted margins, stretching is relatively minor immediately prior to transform breakup, with greater stretching occurring during earlier rift phases.     

Crustal structure of the Whipple Mountains, southeastern California: a refraction study across a region of large continental extension

Summary. Closely spaced refraction profiling across the Whipple Mountains metamorphic core complex in southeastern California yields a complex picture of crustal structure in this region of large continental extension. A NE-directed profile, parallel to the extension direction, reveals a high-velocity mid-crustal layer (6.6–6.8 km s⁻¹) at 16-18 km depth, bounded above and below by laterally discontinuous low-velocity zones (<6.0 km s⁻¹). In marked contrast, a NW-directed profile shows a more uniform 6.0 km s⁻¹ crust down to the crust-mantle boundary. The apparent contrast between these two perpendicular profiles may be related not only to a more complex geologic structure in the NW-SE direction, but also to velocity anisotropy associated with mid-crustal mylonites. Despite the differences between the two refraction profiles, both define a flat Moho at 26-27 km depth with an associated upper mantle-velocity of 7.8 km s⁻¹. This observation is significant as it suggests that, although the amount of extension has been highly variable regionally, the crust is no thinner beneath the Whipple Mountains (where extension has been extreme) than the surrounding mountain ranges. Such an observation requires either that the crust was considerably thicker prior to extension, or that lateral flow in the lower crust and/or inflation of the crust via magmatism occurred contemporaneous with extension.