Acoustic Basement Research Articles

Structure of the basement of the near-Laptev part of the Eurasian basin according to geological and geophysical data

The near-Laptev part of the Eurasian basin is the area of transition from modern spreading to intraplate rifting. We propose an approach to the construction of a 3D model of the basement of the Laptev Sea part of the Eurasian basin based on the linkage of all currently available geological and geophysical data. The structure of the acoustic basement of the Eurasian basin is characterized by alternating troughs and highs in cross-section. The sparse seismic data do not allow us to directly trace the strike of these structures, but they can be correlated with the linearity established by gravity and magnetic data and related to the sequential opening of the basin. For the Laptev Sea part of the Eurasian basin, where linearity is no longer traceable from magnetic data, we propose a method of determining the strike of the basement structures on the basis of seismic stratigraphic analysis. The new 3D model of the acoustic basement in the studied area provided the basis for the tectonic scheme of the entire Eurasian basin. The model reflects the main stages of basin development: continental rifting up to 56 Ma, normal spreading 56–45 Ma, ultra-slow spreading 45–34 Ma, ultra-ultra-slow spreading 34–20 Ma. The southern part of the study area is overlain by a sedimentary cover with an age of 20 Ma and younger, which is associated with the cessation of spreading here no later than 20 Ma.

Holocene paleoenvironmental history of Jackson Lake (Grand Teton National Park, USA) deduced from CHIRP seismic reflection and radiocarbon-dated sediment cores

Sedimentary records from Jackson Lake (Grand Teton National Park, Wyoming, USA) provide an opportunity to investigate the Late Quaternary tectonic and hydroclimatic history of a region known to have considerable earthquake hazard potential. Here, we present a new paleoenvironmental reconstruction for Jackson Lake using high-resolution Compressed High-Intensity Radar Pulse (CHIRP) seismic reflection data and sediment cores from Moran Bay, which is adjacent to the active Teton fault on the southwestern lake margin. Three seismic units (SU) are present within Moran Bay. The lowermost SU-1 defines the acoustic basement, which is characterized by a sharp reflector of variable relief above material with a very low signal-to-noise seismic response; direct sampling reveals this unit to be glaciogenic in origin. SU-2 contains diverse acoustic facies that onlap, downlap, and drape underlying strata. Prominent in SU-2 are faulted, folded, and slumped reflections with mixed amplitude characteristics, as well as reflections organized into prograding clinoforms, which suggest a history of deformation and slope instability in fluvio-deltaic and shallow lacustrine environments. A local unconformity separates SU-2 and SU-3, and above this boundary clinoform packages backsteps towards the hinterland. A bi-directionally tapering package of low-moderate amplitude, continuous to semi-continuous reflections onlap the clinoform; this unit was fully cored (13.8 m), allowing the unit boundary to be sampled and dated. Radiocarbon ages indicate that the core spans ca. 10.4 calibrated kiloyears before present (cal ka BP) to present. Retrogradational stacking of deltaic clinoforms are best explained by increased accommodation in Moran Bay around ca. 10 ka, consistent with a major earthquake and hanging wall subsidence on the Teton fault. Two turbidites identified in the strata align temporally (ca. 10.3 ± 0.4 cal ka BP and ca. 8.3 ± 0.3 cal ka BP) with earthquakes on the Teton fault that have been identified in terrestrial and lacustrine paleoseismic records. From ca. 10.4–6.6 cal ka BP core sediments are relatively rich in clay with highly variable magnetic susceptibility and organic carbon content. Deposition during this period was influenced by an evolving stream network and a relatively dry paleoclimate. From ca. 6.6–2.7 cal ka BP core sediments have higher sand and organic carbon contents, suggesting variable fluvial discharge and climate instability. The youngest core records produced a sharp increase in sedimentation rate, sand and organic carbon content, propelled by greater effective moisture, higher lake levels, and increasing watershed vegetation density. Based on clinoform rollover position, water levels at ∼2,046, ∼2,049, ∼2,053, and 2063 m asl characterized Moran Bay during the Holocene. Together, the seismic and core data records indicate a complex and dynamic Holocene lacustrine evolution controlled by fault motion and water level changes.

Seismo-Stratigraphic Data of Wave-Cut Marine Terraces in the Licosa Promontory (Southern Tyrrhenian Sea, Italy)

Some seismo-stratigraphic evidence on the occurrence of wave-cut marine terraces in the Licosa promontory (Southern Tyrrhenian Sea, Italy) based on Sub-bottom Chirp seismic sections is herein presented. Such evidence is provided by marine terraced surfaces situated at various water depths below sea level and etched into the rocky acoustic basement, which are extensively extending in the seaward extension of the Licosa promontory. It is possible that the isotopic stratigraphy and the terraced marine surfaces are connected, so they can be attributed and dated indirectly. The geologic study of seismic profiles has pointed to the prominence of the acoustic basement, extending to the seabed close to the coast and subsiding seawards under the Quaternary marine succession. Ancient remains of marine terraces, found at a range of water depths between 5 m and 50 m, have documented the major morphological changes of the acoustic basement during the Late Quaternary.

Tectonic-Sedimentary evolution of the Tuscan shelf (Italy): Seismic-stratigraphic/structural analysis of Neogenic succession in the Tyrrhenian Sea between Elba Island and Monte Argentario promontory

A reinterpretation of the vectorized version of public vintage seismic profiles in the Tyrrhenian Sea (Italy), between Elba Island and Monte Argentario promontory, was performed to reappraise the Tuscan shelf tectonic evolution. Despite the almost flat geometry of the seafloor, seismic profiles show a corrugated morphology of the pre-neogenic deformed acoustic basement, organized in structural highs and narrow, mostly N-S and NNW-SSE basins. We identified an intimate relationship between the thrust-related structural highs and the position of the basins, principally located at the forelimb and backlimb of major antiforms, a legacy of a primarily Miocene compressional stage. During the middle Miocene, the Tyrrhenian Sea opening set up, and the extensional front migrated from west to east, progressively activating and deactivating the observed high-angle faults, blandly controlling the sedimentation within the basins. After the late Messinian, a regional collapse stage led to the deepening and widening of the basins. A progressive deactivation of all the normal faults is recorded from the lower Pliocene. After the Late Pliocene/early Pleistocene, the area turned into a passive and widespread sinking stage without any frank tectonic activity. Results show that regional thrusts shaped the main architecture of the Tuscan Shelf shallow crust, while the neogenic depocenters started to develop as thrust-top basins along the flanks of the inherited antiforms. Intriguingly and partially in contrast with previous works, no evidence of low-angle normal fault was observed. We propose an innovative model that poses new questions on the crustal-scale mechanisms responsible for Tyrrhenian extensional process-related features, also establishing a new and unique starting point for fully unraveling the tectonic evolution of this portion of central Italy's offshore domain.

Deep structural controls on the distribution of carbonate reservoirs and overburden heterogeneity in Central Luconia province, offshore Borneo revealed by 3D anisotropic inversion of regional controlled-source electromagnetic and magnetotelluric profile data

Understanding the deep structural controls on reservoir distribution and overburden heterogeneity is important for near-field exploration and green-field development of the Miocene to Holocene carbonate buildups in Central Luconia province in offshore northwest Borneo, Malaysia. Data from 59 stations along an approximately 180 km long controlled-source electromagnetic (CSEM) and magnetotelluric (MT) survey line with three segments recorded using different CSEM transmitter-towing directions were available. We applied three-dimensional (3D) anisotropic resistivity inversion with cross-gradient constraint and verified the resulting models using resistivity logs from nearby wells and the acoustic basement interpreted from seismic data. Our anisotropic resistivity models reveal a fragmented carbonate-rich zone atop a segmented basement comprising electrically resistive horsts coinciding with the northerly Mega-platform, Central and Southern Field Highs, separated by steep conductive zones coinciding with the West, East, and Southeast Troughs previously interpreted from seismic data. The structural highs correlate with the spatial distribution of the known carbonate buildups implying a genetic link. The carbonate bodies are overlain and underlain by persistent layers (C2 and C3) of low resistivity and high anisotropy, which we interpret as indicating compressional deformation or detachment zones. Overburden layer C2 (whose base coincides with the top of a key sedimentary package, Cycle V in seismic data) is thinnest over the East Trough and is discontinuous at the eastern margin of the West Trough, which are locations where drilled wells did not find hydrocarbons implying that the seal rocks are inefficient or breached at those localities. We used these observations to refine the existing seismic-based interpretation of carbonate play-types along our transect indicating how 3D joint CSEM-MT imaging can potentially contribute to derisking or optimizing future exploration and/or development work in this province.

The crustal structure of the western Amundsen Basin, Arctic Ocean, derived from seismic refraction/wide-angle reflection data

SUMMARY Two geophysical expeditions (LOMROG II and III) were carried out in 2009 and 2012 to acquire seismic data in the western Amundsen Basin in the Arctic Ocean, a basin formed by ultraslow seafloor spreading at the Gakkel Ridge. Previous studies show alternating magmatic and amagmatic segments at the ridge but it is unclear if such segmentation persisted throughout the entire opening history of the basin. The seismic refraction data were used to develop P-wave velocity models down to the uppermost mantle using forward modelling of traveltimes. The coincident seismic reflection data were used to constrain the geometry of the sedimentary layers and for characterizing the acoustic basement. 2-D gravity modelling was used to determine the Moho depth in areas when data quality was insufficient to resolve a Moho reflection. The models distinguish three different basement types: oceanic crust with layers 2 and 3, oceanic crust that is lacking a layer 3 and exhumed and serpentinized mantle. The maximum observed crustal thickness is 6 km. Areas with thin crust (<3 km) may be underlain by partially serpentinized mantle. Where exhumed mantle is observed, a serpentinization front separates highly serpentinized mantle at the top from partially serpentinized mantle below. The presence of oceanic crust off-axis of the presently amagmatic sector of the Gakkel Ridge indicates that there is both a spatial and temporal variation of crustal accretion processes at the ridge.

Holocene seismic activity in south‐eastern Switzerland: Evidence from the sedimentary record of Lake Silvaplana

AbstractHigh‐Alpine regions are prone to a large variety of geohazards, among which earthquakes have the strongest impact on landscape and local population. Historic records indicate a moderate to high seismic activity in the northern, south‐western and central parts of Switzerland. In contrast, south‐eastern Switzerland has less historic earthquake chronicles due to the low population density, resulting in a poorly constrained seismic event catalogue. The aim of this study is to evaluate the palaeoseismic activity for south‐eastern Switzerland by using the sedimentary record of Lake Silvaplana in the Engadine Valley. A dense grid of high‐resolution two‐dimensional seismic profiles, high‐resolution bathymetry and a 10 m long sediment core from the deepest basin were used to investigate the stratigraphy of the lake sediments. The bathymetry reveals a flat basin, flanked by steep slopes to the north‐west and south‐east. The acoustic basement consists of four ridges, and gently‐dipping fans to the south‐west and north‐east. Expressions of slope failure can be identified in all domains of the lake floor and the subsurface data. Multiple coevally‐triggered chaotic mass‐flow deposits, overlain by megaturbidites with a coarse‐sand base, have been detected along ten horizons in the seismic data. The four most recent of these deposits are cored and radiocarbon dated to approximately 230, 310, 960 and 1330 cal yr bp, indicating four over‐regional seismic events that triggered large slope failures in Lake Silvaplana in the last 1400 years. Correlation with sediments of Lake Sils, Lake Como, Lake Iseo and Lake Ledro indicate within radiocarbon uncertainties a large earthquake around 1330 cal yr bp. Within their age ranges, the postulated earthquake at 310 cal yr bp (1640 ce) further correlates with a moment magnitude Mw ca 5.4 event in Ftan in 1622 ce, and the 960 cal yr bp (990 ce) earthquake correlates with a Mw ca 5.2 earthquake in Brescia in 1065 ce. Six mass‐movement deposits, also suggested to be caused by earthquakes, were not reached by the sediment core and have suggested ages between 7800 and 11 300 cal yr bp. Thus, Lake Silvaplana sediments provide the first reliable record of seismic activity for the mid and Late Holocene in this region, likely related to the neotectonic activity of the Engadine Line, a major fault zone running along the main valley.

Structural configuration and tectonics of the Flemish Cap, offshore Newfoundland, from newly acquired seismic reflection data

Although the Flemish Cap played a pivotal role in the opening of the North Atlantic, the tectonic history of this continental ribbon has been poorly constrained owing to insufficient seismic coverage. In this study, we present 13 newly acquired seismic reflection profiles over the Flemish Cap, on which seismic reflectors show highly variable seismic facies both on and beneath the top acoustic basement, with exceptional imaging of layered crustal structure. The upper crust is primarily characterized by transparent, chaotic amplitude reflectivity. The lower crust, particularly on the flanks of the cap, exhibits relatively bright and coherent reflection packages interpreted as Appalachian orogenic fabrics based on onshore–offshore correlations from pre-rift plate reconstructions. Extensional systems within the continental crust of the Flemish Cap record a transitional stage between Paleozoic orogenic collapse and pre-Jurassic rifting. The crustal architecture associated with Mesozoic rifting of the Flemish Cap is also mapped and the interpreted distinct rift domains display along-strike variations. Overall, the complex tectonic history of the Flemish Cap involved dominantly ductile deformation during the Paleozoic orogenic stage, multiple deformation styles (primarily ductile and brittle–ductile) during the orogen-to-rift transitional stage and brittle deformation during the major Jurassic–Cretaceous rifting stage.

3D Geophysical and Geological Modeling of the South Orkney Microcontinent (Antarctica): Tectonic Implications for the Scotia Arc Development

AbstractThe opening of the Scotia Arc resulted in the final breakup of the land bridge between South America and the Antarctic Peninsula. The South Orkney Microcontinent (SOM) constituted part of this former connection and it is now the largest continental block in the Southern Scotia Arc. We present the first 3D model of the SOM that, given its strategic position and characteristics, allows us to advance the knowledge of the tectonic processes involved in the development of the Scotia Arc. Due to the scarcity of reliable geological data, the initial approximation of the deep structure of the SOM was supported by the calculation of three main geological boundaries from geophysical data: the acoustic basement, the boundary of the magnetic anomaly source and the Moho depth. The 3D model was built, refined and validated by forward modeling and joint inversion of gravity and magnetic data. We have accurately defined the geometry of the sedimentary cover, determined the geometry of the intrusive igneous body causing the Pacific Margin Anomaly (PMA) and mapped the heterogeneity of the crustal thickness. These structural features show a clear relationship to each other and are consistent with an important E‐W extension to the east of the SOM during early stages of the Scotia Arc formation, prior to the opening of the Powell Basin.

Contrasting lithospheric geophysical structure of the Black Sea subbasins: Relevance to testing geotectonic models for this mega depression

We present an integrated analysis of the geophysical parameters of the lithosphere of Black Sea basin obtained as a result of the interpretation of magnetic, gravity, thermal, deep seismic sounding and seismic-tomographic data. It first demonstrates inherent significant differences in geophysical parameters of lithosphere in the Western and Eastern Black Sea subbasins existing from the prerift stage. The set of reviewed parameters responsible for formation of the present-day lithosphere includes types of crust, depths to acoustic basement, configuration of subbasins, depths to Moho, heat flow and relief of the thermal-asthenosphere boundary (LAB), trends of main deep faults of the crystalline crust, their kinematic types, occurring linear magnetic anomalies, velocity pattern of subcrustal mantle. The above-mentioned parameters of the lithosphere are indicators of the age, geodynamics and driving mechanisms for opening of a subbasins. Oblique trends of the subbasins and the topography of Moho discontinuity in the west and east domains, oblique striking of pre-rifting Istanbul zone and the Shatsky Ridge and different trends of deep faults in the crystalline crust suggest distinct lithospheric structure existing from pre-opening of the Black Sea subbasins and different geodynamical conditions of its formation. The Odesa-Sinop-Ordu deep fault zone as a direct continuation of the Golovanivskaya suture zone of the Ukrainian shield and its slope, probably is of the Precambrian age. So it could be the tectonic boundary between two segments of pre-rift continental crust and between future subbasins. The examples illustrate how indicated parameters of the lithosphere can geophysically confirm the basic ideas of available models for geodynamics of the Black Sea.

Bransfield Strait and South Shetland Islands sedimentary basement and upper crustal structure: An analysis of gravity and magnetic data at the northwestern area of the Antarctic Peninsula

The northwestern region of the Antarctic Peninsula is of main interest to the geosciences. Complex tectonic activity in this area is evidenced by the presence of the sinistral South Scotia ridge system and Shackleton plate contacts; As well as the opening of the Bransfield Strait leading to the formation of the Shetland Block, where the last remnant of the Phoenix convergent margin is located. Potential fields such as gravity and magnetism can be used to study crustal structure in these remote regions, where sedimentary basins are located. In this study, we utilized gravity data from the EIGEN-6C4 combined field model and magnetic data from the ADMAP2 magnetic anomaly model. We employed the Spectral decomposition of gravity anomalies to analyze sedimentary basement depth and directional derivatives of gravity and magnetic anomalies to derive structural features and magmatic activity. The gravimetric basement model was compared with previous interpretations of the acoustic basement and provides a new interpretation of basement morphology in regions that have not been studied previously in detail. The gravity/magnetic derivatives highlight evidence for ongoing magmatic activity in the Central Bransfield Basin which correlates with seismic catalogs, and crustal segmentation between King George and Gibbs Islands related to the unstable intersection of the South Scotia Ridge system and Shackleton Fracture Zone.

Flat Spots Within Cenozoic Sediments of the Nansen Basin, Arctic Ocean: Indicators for Serpentinization, Gas Generation and Accumulation Processes

Flat spot anomalies in the Quaternary part of the section of the Nansen Basin are imaged in seismic records and are interpreted to be related to gas-rich fluid accumulations. The flat spots are mainly located above basement highs between magnetic spreading anomalies C20 (~43 Ma) and C12 (~33 Ma). The complex morphometric analysis of flat spots show that serpentinization processes identified from modelling of gravity anomalies could be original gas source. This process also makes smoothing of the basement highs amplitudes. The depth of the top of the flat spots below the seafloor has an almost constant value of ~390 m indicating the ascent of gases from variable basement depths to a common subsurface fluid trap. The depth of the anomalies below the seafloor corresponds to a theoretical thickness of gas hydrate stability zone in the studied region. Gravity modeling along the Arktika-2011-03 section showed the position of the upper mantle blocks with lower (to 2.95 g/cm3) density within the highs of the acoustic basement. The flat spot anomalies occur above basement highs, below which blocks with lower density typical of serpentinized rocks are modeled. Thus, the serpentinization of the upper mantle ultramafic rocks is considered a main geochemical process, which can explain generation and accumulation of gas in oceanic abyss at a 1–3 km thick sedimentary cover, as well as small vertical movements of the basement blocks due to density reduction and expansion of serpentinized rock.

Southeastern Flank of Knipovich Ridge (Northern Atlantic): Basement Structure and Neotectonics from Geophysical Data and Experimental Modeling

Acoustic basement of the Knipovich Ridge southeastern flank was interpreted on the time-domain CDP seismic sections and calibration of Bouguer gravity anomalies to depth was done with construction of basement structural map for the area with an oceanic crust type. On this map, to the east from Knipovich Ridge, there is the longitudinal uplift, which is the northern continuation of the Senja fracture zone and interpreted as a transverse ridge on the transform fault board. This uplift is framed by linear clusters of the off-axis seismicity epicenters, indicating the activation of this area structures. The CDP seismic data above the identified uplift show deformations of the Pliocene‒Quaternary sedimentary cover with reverse fault and shear kinematics. Physical modeling of structure formation in the area of the Knipovich Ridge clearly demonstrated the fe-atures of the main tectonic elements during oblique spreading. The result, especially close to reality, was obtained by conducting combined experiments with bending the weakened zone to large angles between the direction of stretching and perpendicular to the axis of the weakened zone. At the same time, the a-ppearance of typical accretion swells and nontransform axis displacements simulating the structures of the southeastern flank of the Knipovich Ridge is close to reality. The series of experiments conducted to study the possible formation of the spreading axis jump in an easterly direction to the continuation of the Senja fracture zone showed the fundamental possibility of this structure activation, which we consider as one of the reasons for the formation of features observed in geophysical data. The current position of the active zones of the region, seismicity, the structure of the basement and the structure of the sedimentary cover indicate a shift in the activity of the main tectonic elements in the east direction relative to the current position of the extension axis. The likely scenario for further development of the region will be the transformation of the Knipovich Ridge into one or the series of transform faults parallel to the western edge of the Barents Sea shelf and the series of short spreading segments between them.

Origin of the southern Ryukyu Forearc Terrace: A seaward-protrusion of arc crust elevated by subducted asperities?

• The southern Ryukyu Forearc Terrace exhibits an extremely high free-air gravity anomaly and a thick upper crust. • The rest of the southern Ryukyu forearc region is characterized by prominent forearc basins and a large-volume accretionary prism. • The forearc terrace represents a remnant seaward-protruded arc crust. • The bulged and uplifted basement of the forearc terrace is most probably caused by the subduction of asperities. A forearc terrace with distinct topography and gravity anomalies lies within the Ryukyu forearc between 125°E and 127°E. The origin and tectonic affinity of this terrace remain uncertain. This study attempts to depict the structural characteristics of the forearc terrace and to trace its origin and formation mechanisms through a comprehensive examination of newly acquired geophysical data combined with the results of previous surveys. The forearc terrace exhibits shallow bathymetry, an extremely high free-air gravity anomaly, a shallow buried acoustic basement, a thin sedimentary cover, and a thick upper crust. In contrast, the rest of the southern Ryukyu forearc region is characterized by a steep upper forearc slope, prominent forearc basins, a deeply buried acoustic basement, and a large-volume accretionary prism. The forearc terrace represents a remnant seaward-protrusion of arc crust created after the frontal portion of the arc crust had been removed by subducted ridges or other types of linear bathymetric highs. This is in contrast to other formation mechanisms such as a block sliding seaward along strike-slip faults or an accretion of exotic terrane from the outer margin of the original arc crust after being scraped off from the subducting oceanic crust. The bulged and uplifted basement of the forearc terrace is most likely associated with subducted seamounts. Further studies, especially deep-sea drilling, are necessary to verify the nature and origin of the forearc terrace.

Neogene-Quaternary evolution of the La Baja Guajira basin, Colombia: Burial and thermal history with implications on petroleum systems

The La Baja Guajira Basin is South America's northernmost part that extends from onshore Colombia into the Caribbean Sea and represents an important gas-producing region in Colombia. The dataset used in this study consists of twenty-four (24) regional 2D seismic-reflection lines integrated with information from nine (9) boreholes across the basin. We present seismic interpretations of key stratigraphic and structural features constrained by 1D basin modeling. This allows us to describe the Neogene-Quaternary evolution of the basin, and understand the possible implications for the hydrocarbon potential.We have interpreted eight (8) seismic units within the La Baja Guajira Basin (acoustic basement to U7) affected by three fault groups: 1) NW-SE-striking basement normal faults, 2) inverted normal faults, and 3) strike-slip faults. Considering the evidence, we propose four tectonic phases along the basin's evolution. The basement was affected by normal faulting as the result of extensional tectonics prior to the deposition of the sedimentary infill (Paleogene?). Another extensional interval occurred during the Early Miocene, triggering a moderate accommodation within the proximal area. Subsequently, extension migrated westwards resulting in the formation of the main La Baja Guajira Basin depocenter. In the Middle Miocene (post-extension), sedimentation was controlled by exhumation pulses in adjacent areas to the basin, which resulted in significant sediment input contemporaneous with rapid tectonic subsidence. Yet, local inversion of NW-SE-oriented basement-rooted faults affected today's offshore domain until the Late Miocene. This inversion can be associated with regional compressive deformation linked to the activity of the Oca and Cuiza faults. A second episode of rapid tectonic subsidence happened in the Early Pliocene. In the south of the basin, rocks were subjected to maximum burial and highest temperatures in recent times, conditions that might lead to hydrocarbon generation. The high Neogene to recent sedimentation rates calculated for the La Baja Guajira Basin support microbial gas generation; structural reactivation in this time supported trap formation, particularly in the offshore parts of the basin.

Structural and tectonic conditions for the development of the Bering Sea sedimentary basin

Introduction. In order to characterize the oil and gas potential of any area under exploration, its structural and tectonic evolution should be studied. In this paper, a structural and tectonic modelling of the Bering Sea is carried out.Aim. To identify the structural and tectonic characteristics of the Bering Sea by interpreting the results obtained during a geodynamic analysis of sedimentary basin formation, based on the tectonic and geodynamic paleoreconstructions and sedimentary basin modelling of the Bering Sea.Materials and methods. A structural and tectonic modelling of the Bering Sea sedimentary basins was carried out using contemporary methods of basin analysis and numerical geological modelling (PetroMod software, Schlumberger). Three-dimensional time-spatial structural-tectonic models of the Bering Sea were formed using the bottom structural maps of Pliocene-Quaternary deposits, near the Lower Miocene and Oligocene tops and along the acoustic basement bottom. Maps were digitalized and converted to grids (with a 500-m step), in which the discrepancies (intersections) were removed taking into account the available geological and geophysical data (seismogeological sections). The contemporary surface of sedimentary basins was constructed by the connection of bathymetric and topographic maps. The beginning and end time of sedimentary accumulation periods was determined in accordance with the international stratigraphic scale.Results. The performed study identified the sufficiently continuous development areas of the oceanic or suboceanic crust of deep-water (back-arc) basins, aged from the Upper Jurassic-Cretaceous to the Cenozoic and repeatedly affected by the subsequent phases of the tectonic and magmatic activation; development belts of the Cretaceous-Cenozoic block-magmatic basement of island arcs, locally including reformed basement blocks of an older, Paleozoic or Cimmerian, consolidation; extensive depth-differentiated alpine/newest (syn-oceanic) shelf platforms, occassionally partially destroyed due to the latest destruction, including blocks or large blocks of Pre-Cambrian or Paleozoic relatively rigid massifs in the structure of their base.Conclusion. The modelling results indicate the deeply submerged West Anadyr, East Anadyr and Central Anadyr basins to be possible depocentres with their own hydrocarbon generation centres.

Geophysical insights on the crustal structure of Greenland's northern continental margin towards the Morris Jesup Spur

During expedition PS115/1, the German research vessel Polarstern acquired seismic refraction data along a 102-km-long profile crossing Greenland's northern continental margin and extending up to the southwestern limit of the Morris Jesup Spur. A P–wave velocity model is obtained and validated by gravity modelling. A nearby seismic reflection line provides insights on the structures within the sedimentary cover. Beneath a 2-km-thick sedimentary cover with velocities of 1.8 km/s to 3.4 km/s, an up to 1.5-km-thick layer is characterized by velocities of 4.2 km/s and is interpreted to consist of volcanic rocks. This is consistent with proposed volcanic activity on the Morris Jesup Spur and exposed volcano-sedimentary rocks of the nearby Kap Washington Group. Below the volcanic rocks, an up to 7 km-thick unit with velocities of 4.4 to 5.8 km/s is interpreted to consist of metasedimentary rocks still belonging to the deformed units of the Franklinian Basin. The seismic reflection data image a tectonic overprint of the acoustic basement and the lowermost sedimentary layers. Tectonic faults indicating the tectonic overprint do not reach to the uppermost sedimentary layers. This hints at an older fracture zone that is also observed in the velocity model and aligns well with observed anomalies in the regional gravity and magnetic fields. The seismic velocity model reveals a highly extended continental crust that shows a magmatic overprint at all crustal levels. The upper crust is 5 km thick with a velocity of 6.0 km/s. However, in three distinct zones its thickness is up to 8 km and the velocity increases to 6.3 km/s. These zones are interpreted as magmatic intrusions into the upper crust. The lower crust represents a 9-km-thick high-velocity layer (7.2 km/s) that is interpreted as magmatic underplating or lower crustal sill intrusions. Such a high-velocity lower crust is not present in models of adjacent Arctic margins.PACS: 0000, 11112000 MSC: 0000, 1111

Subsurface structure of the Mazatlán basin, southeastern Gulf of California, Mexico, from seismic reflection and well data

The structure of the southeastern Gulf of California rifted margin is only partly known because of the thick sedimentary cover of the continental shelf and coastal areas of Sinaloa and Nayarit. Here we report the subsurface structure and probable age of the Mazatlán basin, one of the largest buried basins, based on interpretation of reprocessed by multichannel 2D reflection seismic profiles and an exploratory well drilled by Petroleos Mexicanos (PEMEX) between 1979 and 1980, as well as gravimetric data. The Mazatlán basin is located just north of the active subduction zone of the Rivera plate and west of the new oceanic crust accreted since the Pliocene at the mouth of the Gulf. The basin is bounded to the west by a high of the Late Cretaceous batholiths exposed in the Tres Marias Islands and in the normal faults bounding the Nayarit Troughs, and to the east by the late Oligocene-early Miocene extensional faults bounding the Sierra Madre Occidental. Based on the characteristics of the seismic profiles, the basin can be divided into three main sectors. The transition between the northern and central sectors is defined by an abrupt deepening of the basement that corresponds to the westward prolongation of ENE-WSW trending accommodation zones separating opposite tilted domains in the Sierra Madre Occidental. The acoustic basement is clearly defined in the central sector by a set of high-amplitude reflectors suggesting an igneous origin. The deepest part of the basin is located toward the coastal plain of Nayarit and coincides with a gravimetric minimum. We identify several high amplitude reflectors interpreted as mafic sills, which become more abundant toward the south. At least part of them can be correlated with the widespread mafic volcanism emplaced at 9–12 Ma along the eastern margin of the Gulf of California rift. In the northern and central sectors we identified three main stratigraphic boundaries (L1, L2, and L3) within the basin filling, which define seismo-stratigraphic sequences. The shallower sequences, limited by the L1 and L2 boundaries and attributed to the Pleistocene and part of the Pliocene, lack evidence of significant deformation. The sequence above L3 boundary, can be assigned to the earliest Pliocene and the Miocene since they are intruded by a ~ 10 Ma basaltic sill cut by the Huichol-1 exploratory well. The lower part of the basin is characterized by high amplitude reflectors that pinches out against the acoustic basement and correspond to volcanic tuffs and sedimentary deposits correlative with the lower member of the Comondú group exposed in Baja California, which includes ~23 to ~19 Ma distal part of ignimbrites sourced from the Sierra Madre Occidental in Nayarit. Faults with the most significant displacements have a listric geometry and dip from the E to NE. The maximum sediment thickness reported in the Huichol-1 well is 2800 m, with a two-way travel time of 2.7 s, but it reaches 3.7 TWTT in some areas. Based on sedimentation rates, the minimum age for the onset of sedimentation in the basin is ~19 Ma, which is consistent with the regional tectonic evolution of the margin.

Back-barrier evolution and along-strike variations in infilling of the Kosi Bay lake system, South Africa

This study examines both the inlet dynamics, recorded in the back-barrier stratigraphy of Kosi Bay, and the competing effects of waterbody segmentation on along-strike changes in the character and nature of an incised valley fill at Kosi Bay, South Africa. The Kosi Bay system comprises four interconnected lakes (Lake Makhawulani, Lake Mpungwini, Lake Nhlange and Lake Amanzimnyama) subject to varying degrees of marine influence. Formerly, the system's only connection to the ocean was through the Bhanga Nek palaeo-inlet. The location of this system behind a continuous coastal barrier predisposes it to excellent archives of environmental change related to changing sea-level and sediment supply. In this context, two cores were extracted from the system (KB2 and KB4), together with over 150 km of seismic reflection profiling, as well as AMS radiocarbon analyses and legacy data in order to present a new stratigraphic evolution model for this back-barrier system. A total of 5 seismic units were imaged (Units A-E). A major unconformity surface (SB), characterised by broad u-shaped incised valleys, incises the underlying unconsolidated sandy acoustic basement (Unit A). Unit B forms as thickly developed central basin deposits, partially infilling the incised valleys. Two forms of prograding spits are evident -Unit C1 progrades from the palaeo-highs of the valley interfluves into the nearest available accommodation space and Unit C2 progrades from the margins of the system into the basin. These are separated by an erosional surface (T-RS) formed by migrating tidal channels as sea-level rose and marine waters entered the system during the Holocene. Unit C2 spits are in turn associated with slump deposits formed on the steepest part of the margins (Unit D) that were dated to 2900 ± 165 cal. BP. The unusual depth of Surface T-RS is attributed to (1) limited sediment supply during sea-level rise, (2) the presence of easily erodible fine sediments in which this surface erodes and (3) a larger tidal prism when the Bhanga Nek inlet was still open and system segmentation by Units C2 and D had not yet occurred. Unit E caps the stratigraphy and is interpreted as lacustrine fines with a thin layer of acoustically transparent material (gyttja) on top. The deposition of Unit E signifies the closure of the system to the ocean and the overall shift from an estuarine/lagoon environment to a lacustrine one. Recalibrated AMS dates place the system closure to between 3160 and 2900 cal. BP, coeval with the formation of the prograding marginal spits and waterbody segmentation (Unit C2). Along-strike variation in the timing of the basin infilling, attributed to changes to the inlet functioning coupled to the segmentation of the waterbody, is characteristic of this area. At 1450 cal. BP, a new tidal inlet formed to the north of Lake Makhawulani, where impounded waters were diverted via a low point in the back-barrier coastal dune cordon. The formation of this inlet suggests a prolonged period of back-barrier flooding until a low point in the barrier was breached during the Holocene sea-level highstand of 1.5 m. Such inlet development and along-strike back flooding of incised valleys is an unusual attribute of coastal waterbodies of South Africa.

The crustal structure and evolution of the Bunguran Trough, offshore Sarawak, Malaysia

The Bunguran Trough is a deep Tertiary basin depocentre in the West Luconia Delta Province, Sarawak Basin, offshore Malaysia. At the centre of the trough, the top of the acoustic basement is ∼15 km deep, buried beneath a thick pile of Late Eocene to Quaternary coastal-deltaic sediments of the proto-Rajang-Lupar River delta system. Sediment thickness, bathymetry and gravity data were utilized to re-construct the crustal structure of the West Luconia Delta and the Bunguran Trough. The results indicate an average crustal thickness of 5.3 km beneath the Bunguran Trough, which is extremely thin compared to the extended continental crust (∼15–22 km) beneath the adjacent West, Central and North Luconia provinces. Extreme crustal thinning is confined to the Bunguran Trough, which appears to have steeply faulted boundaries, but the nature of the faults is obscured by the thick sedimentary pile.Structural evidences from reflection seismic data suggest widespread extensional faulting throughout Sarawak Basin although some major tectonic lineaments may have had significant strike-slip movements due to later reactivation and basin inversion. Both extensional and strike-slip faulting may have contributed to the formation and subsidence of the Bunguran Trough. West Balingian Line is a major fault zone that seems to have had a significant strike-slip displacement history and probably had an important role during basin initiation. Seismic data suggests that this fault zone extends from the Balingian and Tatau provinces into West Luconia as a major strike-slip fault with a releasing bend along which the Bunguran Trough may have developed. Crustal extension in a pull-apart basin may have been initiated by sinistral motion along the fault during Late Eocene-Oligocene. Alternatively, the widespread normal faulting indicates that the Bunguran Trough may have developed as a rift basin which experienced at least two phases of extension before a major inversion event which is marked by the Middle Miocene Unconformity (MMU). The main phase of subsidence in the Bunguran Trough had occurred during the active extensional phase before the MMU.