Multichannel Seismic Data Research Articles

Shallow subsurface morphotectonics of the NW Sumatra subduction system using an integrated seismic imaging technique

In subduction zones, shallow subsurface structures are the manifestation of the plate interactions at depth. However, significant water depths, rough bathymetry and presence of heavily deformed accretionary wedge materials hamper imaging of the near-surface features to a great extent using conventional imaging techniques. In this study, we show results using an integrated processing technique to a multichannel seismic data set acquired in 2006 from the northwestern offshore Sumatra. We start with first downward continuing the 12-km-long surface streamer data to the seafloor, followed by a high-resolution traveltime tomography of refracted phases to determine a detailed velocity–depth model of subsurface, which in turns, is used for pre-stack depth migration in order to delineate the shallow subsurface structures beneath the trench, subduction front and outer accretionary wedge. Our velocity–depth model and the depth migrated image depict variation of sediment properties across the front and structures of uppermost sedimentary sequence with an unprecedented high resolution providing the precise location of the frontal and conjugate thrusts, highly folded sedimentary sequences, which in turns describe their relationship with the top of the subducting plate and factors that control rupture propagation to the trench. Furthermore, we estimate the porosity distribution across the front, where we find a 12 and 18 per cent decrease in porosity beneath the deformation front and the inner accretionary plateau at 500 m below the seafloor, respectively, which we interpret to be due to the compaction. A significant decrease in porosity at the plate interface below 5–6-km thick sediments near the deformation front would increase the coupling, leading to the rupture propagation up to the trench, uplifting 4.5 km water and producing large tsunami.

Characteristics and occurrence of submarine canyon-associated landslides in the middle of the northern continental slope, South China Sea

High-resolution and high-density 2-D multichannel seismic data, combined with high-precision multibeam bathymetric map, are utilized to investigate the characteristics and distribution of submarine landslides in the middle of the northern continental slope, South China Sea. In the region, a series of 19 downslope-extending submarine canyons are developed. The canyons are kilometers apart, and separated by inter-canyon sedimentary ridges. Numerous submarine landslides, bounded by headscarps and basal glide surfaces, are identified on the seismic profiles by their distorted to chaotic reflections. Listric faults and rotational blocks in head areas and compressional folds and inverse faults at the toes of the landslides are possibly developed. Three types of submarine landslides, i.e., creeps, slumps, and landslide complexes, are recognized. These landslides are mostly distributed in the head areas and on the flanks of the canyons. As the most widespread landslides in the region, creeps are usually composed of multiple laterally-coalesced creep bodies, in which the boundaries of singular component creep bodies are difficult to delineate. In addition, a total of 77 landslides are defined, including 61 singular slumps and 16 landslide complexes that consist of two or more component landslides. Statistics show that most landslides are of a small dimension (0.53–18.09 km² in area) and a short runout distance (less than 3.5 km). Regional and local slope gradients and rheological behavior of the displaced materials might play important roles in the generation and distribution of the submarine landslides. A conceptual model for the co-evolution of the canyons and the associated landslides in the study area is presented. In the model it is assumed that the canyons are initiated from gullies created by landslides on steeper sites of the continental slope. The nascent canyons would then experience successive retrogressive landsliding events to extend upslope; at the same time canyon downcutting or incision would steepen the canyon walls to induce more landslides.

The Design of Multichannel Marine Seismic Acquisition Unit Based on ADS1282

For marine seismic data acquisition needs,a multichannel marine seismic data acquisition unit was designed,which used the 32-bit analog-to-digital ADS1282 as a core and Field programmable gate array (FPGA) as the acquisition controller.The unit can achieve multichannel seismic data sampling and transmission functions.The design fully used with the design ADS1282 chip integration,with the corresponding anti-jamming measures,not only simplified the circuit design,but also ensured the quality of signal acquisition and system stability.the design used FPGA to realize a multichannel hydrophone signal synchronization sampling.

Seismostratigraphy of the Siberian Sector of the Arctic Ocean and adjacent Laptev Sea Shelf

AbstractA new seismostratigraphic model has been established within the Arctic Ocean adjacent to the East Siberian Shelf on the basis of multichannel seismic reflection data acquired along a transect at 81°N. Ages for the sedimentary units were estimated via links to seismic lines and drill site data of the US Chukchi Shelf, the Lomonosov Ridge, and the adjacent Laptev Shelf. Two distinct seismic units were mapped throughout the area and are the constraints for dating the remaining strata. The lower marker unit, a pronounced high‐amplitude reflector sequence (HARS), is the most striking stratigraphic feature over large parts of the Arctic Ocean. It indicates a strong and widespread change in deposition conditions. Probably, it developed during Oligocene times when a reorientation of Arctic Plates took place, accompanied by the gradual opening of the Fram Strait, and a widespread regression of sea level. The top of the HARS likely marks the end of Oligocene/early Miocene (23 Ma). An age estimate for the base of the sequence is less clear but likely corresponds to base of Eocene (˜56 Ma). The second marked unit detected on the seismic lines parallels the seafloor with a thickness of about 200 ms two‐way travel time (160 m). Its base is marked by a change from a partly transparent sequence with weak amplitude reflections below to a set of continuous high‐amplitude reflectors above. This interface likely marks the transition to large‐scale glaciation of the northern hemisphere and therefore is ascribed to the top Miocene (5.3 Ma).

Ice-load induced tectonics controlled tunnel valley evolution – instances from the southwestern Baltic Sea

Advancing ice sheets have a strong impact on the earth's topography. For example, they leave behind an erosional unconformity, bulldozer the underlying strata and form tunnel valleys, primarily by subglacial melt-water erosion and secondarily by direct glacial erosion. The conceptual models of the reactivation of faults within the upper crust, due to the ice sheets' load, are also established. However, this phenomenon is also rather under-explored. Here, we propose a causal link between ice-load induced tectonics, the generation of near-vertical faults in the upper crust above an inherited deep-rooted fault and the evolution of tunnel valleys. The Kossau tunnel valley in the southeastern Bay of Kiel has been surveyed by means of high-resolution multi-channel seismic and echosounder data. It strikes almost south to north and can be mapped over a distance of ca 50 km. It is 1200–8000 m wide with a valley of up to 200 m deep. Quaternary deposits fill the valley and cover the adjacent glaciogenic unconformity. A near-vertical fault system with an apparent dip angle of >80°, which reaches from the top Zechstein upwards into the Quaternary, underlies the valley. The fault partially pierces the seafloor and growth is observed within the uppermost Quaternary strata only. Consequently, the fault evolved in the Late Quaternary. The fault is associated with an anticline that is between 700 and 3000 m wide and about 20–40 m high. The fault–anticline assemblage neither resembles any typical extensional, compressional or strike-slip deformation pattern, nor is it related to salt tectonics. Based on the observed position and deformation pattern of the fault–anticline assemblage, we suggest that these structures formed as a consequence of the differential ice-load induced tectonics above an inherited deep-rooted sub-salt fault related to the Glückstadt Graben. Lateral variations in the ice-load during the ice sheet's advance caused differential subsidence, thus rejuvenating the deep-rooted fault. As a result, the inherited fault propagated upwards across the Zechstein and post-Permian overburden and further grew during the ice sheet's retreat. The developing fault and anticline system under the ice sheet created a weakness zone that facilitated erosion by pressurized glacial and subglacial melt-water, as well as by the glaciers themselves. Near-vertical faults cutting through the post-Permian are abundant in the southwestern Baltic realm, which implies that the ice-load induced tectonic activity described above was not an isolated incident.

Estimation of seismic attenuation of gas hydrate bearing sediments from multi-channel seismic data: A case study from Krishna–Godavari offshore basin

Seismic attenuation of gas hydrate bearing sediments is estimated from the multi-channel seismic data acquired in Krishna–Godavari (KG) offshore basin in the vicinity of Site NGHP-01-10, where ∼128 m of fracture-filled hydrate is confirmed by drilling/coring, to understand the effect of gas hydrate on the seismic attenuation. The seismic quality factor (Q), which is the inverse of seismic attenuation, is estimated from two methods – first, the spectral ratio method which utilizes the logarithmic ratio of the seafloor and bottom simulating reflector (BSR) spectra; and second, the peak frequency method which depends upon the variations of the peak frequency of the BSR spectrum as a function of traveltime. In the study area, the complex fault system has perturbed the seafloor resulting in noisy seafloor spectrum. As a result, the logarithm of the spectral ratio was also noisy and the Q-values obtained from spectral ratio method were found to be unstable. On the other hand, the peak frequency method is found to be relatively stable as it depends only on the peak frequency of the BSR which can be reliably obtained by conventional processing. This method yields an effective Q value of BSR which represents a weighted average of the interval Q-values of the water column and the gas hydrate bearing sediments. Since the Q-value of water column can be treated as a constant, the variations in effective Q-value can be interpreted in terms of the variations of Q in the gas hydrate bearing sediments for a constant thickness of water column.The estimated effective Q-values, along the inline and crossline seismic profiles, depend on several factors such as gas hydrate, free gas and the complex fault system. The combined interpretation of the quality factor and the interval velocity model helps to understand these effects in different zones of the seismic data. The attenuation of the background marine sediments is estimated from the analysis of crossline seismic profile. The gas hydrate bearing sediments, as inferred from the velocity model, show low attenuation as compared to that of the background sediments. In contrast, active migration of free gas across the fault system leads to increase in seismic attenuation within the gas hydrate stability zone. The complex fault zone may also lead to an increase in seismic attenuation. The blanked seismic zone shows a marginal decrease in seismic attenuation probably related to load-bearing form of gas hydrate. Our attenuation results are compared against the results obtained from waveform inversion, albeit with a higher resolution. The relative variation of Q estimated from the proposed method is close to the average Q-values estimated from full waveform inversion for most of the CDPs. The proposed methodology can be a useful technique to estimate seismic attenuation from multi-channel seismic data.

Seismic structure of the north‐central Chilean convergent margin: Subduction erosion of a paleomagmatic arc

AbstractWe study the erosive convergent margin of north‐central Chile (at ~31°S) by using high‐resolution bathymetric, wide‐angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2‐D velocity‐depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0–6.5 km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5–5.0 km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (~1 km offset), which overlies a strong lateral velocity contrast from ~5.0 to ~6.0 km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic‐continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large‐scale crustal thinning and long‐term subsidence of the outermost fore arc.

Blind deconvolution of multichannel recordings by linearized inversion in the spectral domain

In seismology, blind deconvolution aims to recover the source wavelet and the Green’s function, or parts of it (e.g., reflectivity series), from a recorded seismic trace. A multitude of algorithms exist that tackle this ill-posed problem by different approaches. Making assumptions on the phase spectra of the source wavelet and/or the statistical distribution of the reflectivity series is useful for single trace. The nature of closely spaced multichannel recordings enables a better estimation of a common source wavelet and thus increases the confidence of the results. This approach has been exploited in the past, although different types of assumptions are used for a variety of algorithms. We introduced a new method for simultaneous reconstruction of arbitrary source wavelets and local vertical reflectivity series from teleseismic earthquakes. Closely spaced receivers record vertically incident earthquake body waves and their surface-related multiples, which comprise the unknown reflectivity series. By assuming a common source wavelet for all receivers, the observation of several events resulted in a set of convolution equations relating the unknown source wavelets and unknown reflectivity series to the observed seismic trace. The overdetermined system of equations was linearized and solved by conventional inversion algorithms in the spectral domain. Synthetic tests indicated a better performance of the introduced method than conventional deconvolution in the presence of white noise, which is attributed to the constraint of a common model for all observations. Application to field data from a local deployment allowed imaging a basement reflector from teleseismic body waves, although the data were contaminated with strong coherent noise. From a practical point of view, the presented method is potentially well suited for local and regional large-scale imaging from multichannel passive seismic data.

Basement characteristics along South West Indian Margin

Abstract Results from detailed analyses of multi-channel seismic reflection data in association with gravity anomalies and lithology data from drilled holes are used to find out the basement structure in the vicinity of the Laccadive Ridge and Laccadive basin along southwest Indian margin. Morphology and seismic interpretation results show the basement in the study area having faulted blocks with horst/graben pattern, and there are basement highs parallel to the coastal line. While the margin highs are sediment deprived, thick sediments are observed in surrounding areas of the margin highs, with thickness up to 2-3s (TWT), and these sedimentary layers usually pinch out suddenly at the lower wall of steep normal faults. A thin layer of sediments is observed on the Laccadive Ridge in this region which has several small ridge peaks (distributed along ridge extension direction), that sometimes outcrop the seafloor. The basement ridge peaks are usually several thousand meters wide, and show obvious high abnormal gravity anomaly. The basement in the Laccadive basin is highly faulted, and appears to has evolved as stretched margin.

Tectonic and sedimentary processes along the ultraslow Knipovich spreading ridge

2D multichannel seismic data and bathymetric records from the glaciated western Svalbard margin and the rift valley region of the ultraslow, and oblique-spreading, Knipovich Ridge are in this study interpreted to infer differences in seafloor spreading mechanisms and to identify sedimentary processes. Our results show that the rift flank geometry, the rift valley elevation and the active magmatism are closely linked. The inferred magmatic segments of the Knipovich Ridge exhibit high and steep rift flanks, whereas the rift flank heights of the proposed tectonic-dominated segments are lower and less steep. In addition, we observe significant rift flank asymmetry across the rift valley which can be partly explained by subsidence due to sediment loading. The identification of a huge sedimentary wedge on the western rift flank suggests that the oldest parts of these sediments have been transported from the western Svalbard margin and across the rift valley. However, we suggest that most of these sediments are glacimarine/hemipelagic sediments which have been deposited in the time period after the rift valley flanks had developed sufficiently to cut off the direct transport routes from the western Svalbard margin. We also observe thick current depositions on the western side, suggesting a strong along-slope influence of the West Spitsbergen Current during the Plio–Pleistocene time period.

Detection and mapping of cold-water coral mounds and living Lophelia reefs in the Galicia Bank, Atlantic NW Iberia margin

We report, for first time, the occurrence of cold-water coral mounds and large number of living Lophelia reefs along the Galicia Bank (Atlantic NW Iberian margin). Detection and mapping of living coral reefs and mounds have been carried out by means of multibeam bathymetry, backscatter images, ultra-high resolution and high-resolution multichannel seismic reflection data and sampling. In addition, profiling CTDs have been made to characterise environmental conditions as temperature, salinity and dissolved oxygen contents of the coral sites. Based on these dataset, two main provinces of cold-water coral mounds and reefs have been identified in the Galicia Bank at water depths between 620 and 1125 m. The Breogham Mound Province along the western flank of the Galicia Bank shows a sequence of well-developed mounds composed of: (1) Stepped semi-buried elongated mounds, the tallest mounds with heights up 70 m and widths of 450 m lined up in along-slope trending ridges and intercalated with upslope migrating sediment drifts at water depths from 1125 to 826 m; (2) Single and composite seabed mounds show heights of 10–12 m and average widths of 100 m at 825 and 780 m water depths and (3) Lophelia reefs forming mini-mounds 2–4 m high and 80–100 m width growing on a flat erosional surface at the summit of the bank at 780–750 m water depths. Otherwise, the Castelao Mound Province at the eastern flank of the bank is mainly constituted by cluster of mini-mounds hosting living Lophelia pertusa and Madrepora oculata reefs. These living reefs are detected based on ultra-high resolutions parametric echosounders as mini-mounds with an acoustically transparent interior. Otherwise, semi-buried and exposed mounds are mapped with a combination of multibeam and backscatter images identified by their high strength values and seabed expression. We conclude that the initiation, growth and distribution of cold-water coral mounds on the Galicia Bank have been controlled by the following environmental forcing factors: (i) water masses with a potential density resulted of the turbulent boundary between the Mediterranean Outflow and the intermediate Atlantic water masses which allowed the transport of coral larvae along the NE Atlantic margins (ii) the formation of a hard substrate, possibly related to subsurface fluid flow, necessary for the initial settle of coral larvae and (iii) the turbulences created by the Mediterranean flow impinging the topography of the bank creating suitable currents that favour intrusion of the nutrient afflux from the Atlantic mass waters to deep waters allowing to coral colonies to growth.

Seabed fluid expulsion along the upper slope and outer shelf of the U.S. Atlantic continental margin

AbstractIdentifying the spatial distribution of seabed fluid expulsion features is crucial for understanding the substrate plumbing system of any continental margin. A 1100 km stretch of the U.S. Atlantic margin contains more than 5000 pockmarks at water depths of 120 m (shelf edge) to 700 m (upper slope), mostly updip of the contemporary gas hydrate stability zone (GHSZ). Advanced attribute analyses of high‐resolution multichannel seismic reflection data reveal gas‐charged sediment and probable fluid chimneys beneath pockmark fields. A series of enhanced reflectors, inferred to represent hydrate‐bearing sediments, occur within the GHSZ. Differential sediment loading at the shelf edge and warming‐induced gas hydrate dissociation along the upper slope are the proposed mechanisms that led to transient changes in substrate pore fluid overpressure, vertical fluid/gas migration, and pockmark formation.

Seismic imaging and velocity structure around the JFAST drill site in the Japan Trench: low Vp, high Vp/Vs in the transparent frontal prism

Seismic image and velocity models were obtained from a newly conducted seismic survey around the Integrated Ocean Drilling Program (IODP) Japan Trench Fast Drilling Project (JFAST) drill site in the Japan Trench. Pre-stack depth migration (PSDM) analysis was applied to the multichannel seismic reflection data to produce an accurate depth seismic profile together with a P wave velocity model along a line that crosses the JFAST site location. The seismic profile images the subduction zone at a regional scale. The frontal prism where the drill site is located corresponds to a typically seismically transparent (or chaotic) zone with several landward-dipping semi-continuous reflections. The boundary between the Cretaceous backstop and the frontal prism is marked by a prominent landward-dipping reflection. The P wave velocity model derived from the PSDM analysis shows low velocity in the frontal prism and velocity reversal across the backstop interface. The PSDM velocity model around the drill site is similar to the P wave velocity model calculated from the ocean bottom seismograph (OBS) data and agrees with the P wave velocities measured from the core experiments. The average V p/V s in the hanging wall sediments around the drill site, as derived from OBS data, is significantly larger than that obtained from core sample measurements.

Seismic structure of the Central Tyrrhenian basin: Geophysical constraints on the nature of the main crustal domains

AbstractIn this work we investigate the crustal and tectonic structures of the Central Tyrrhenian back‐arc basin combining refraction and wide‐angle reflection seismic (WAS), gravity, and multichannel seismic (MCS) reflection data, acquired during the MEDOC (MEDiterráneo OCcidental)‐2010 survey along a transect crossing the entire basin from Sardinia to Campania at 40°N. The results presented include a ~450 km long 2‐D P wave velocity model, obtained by the traveltime inversion of the WAS data, a coincident density model, and a MCS poststack time‐migrated profile. We interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity‐derived density models with existing compilations for continental crust, oceanic crust, and exhumed mantle. The first domain includes the continental crust of Sardinia and the conjugate Campania margin. In the Sardinia margin, extension has thinned the crust from ~20 km under the coastline to ~13 km ~60 km seaward. Similarly, the Campania margin is also affected by strong extensional deformation. The second domain, under the Cornaglia Terrace and its conjugate Campania Terrace, appears to be oceanic in nature. However, it shows differences with respect to the reference Atlantic oceanic crust and agrees with that generated in back‐arc oceanic settings. The velocities‐depth relationships and lack of Moho reflections in seismic records of the third domain (i.e., the Magnaghi and Vavilov basins) support a basement fundamentally made of mantle rocks. The large seamounts of the third domain (e.g., Vavilov) are underlain by 10–20 km wide, relatively low‐velocity anomalies interpreted as magmatic bodies locally intruding the mantle.

Tectonic contrast between the conjugate margins of the South China Sea and the implication for the differential extensional model

The extensional model of the South China Sea (SCS) has been widely studied, but remains under debate. Based on the latest high-quality multi-channel seismic data, bathymetric data, and other obtained seismic profiles, the asymmetric characteristics between the conjugate margins of the SCS are revealed and extensional model of the SCS margin is discussed further. Spatial variation of morphology, basement structure, and marginal faults are discovered among the SCS margin profiles. As for the NS-trending variation, the basement of northern margin displays in the shape of step downwards to the sea, while the basement of southern margin is composed of wide rotated and tilted blocks, without any obvious bathymetric change. The variation also exists in the development of marginal faults between the conjugate margins, and detachment fault system is identified on the southern margin. Along the southern margin from east to west, the Eastern and Southwestern Basins developed different structural units. Based on the tectonic contrast of the conjugate margins, differential extensional model is proposed to explain the spatial variation of the SCS structure, which introduces detachment faults controlling the evolution of the SCS. The upper crust above the detachment fault was deformed by simple shear, while the lower crust and upper mantle below the detachment fault was deformed by pure shear. Because of the different lateral transfer between the upper brittle faulting and the lower ductile extensional regions, there developed marginal plateau (Liyue basin) and outer rise (Zhenghe massif) on the lower plate margin of the Eastern Basin and the Southwestern Basin, respectively. The evolution of the present SCS may be influenced by the diachronous close of the paleo-SCS.

Geological structure of the offshore Sumatra forearc region estimated from high-resolution MCS reflection survey

To investigate detailed fault distributions and shallow geological structure offshore northwestern Sumatra, we obtained high-resolution Multi-Channel Seismic (MCS) reflection data around the Sunda Trench, trench slope, and forearc high regions offshore northwestern Sumatra. In general, trench-parallel anticlinal ridges are distributed from trench slope region to forearc high region. Two kinds of different vergence systems are characterized in the Sumatra forearc region; landward vergence is dominant in the lower trench slope region, and seaward vergence is dominant in the forearc high region. Moreover, piggyback or slope basins are recognized between anticlinal ridges. Deformation in the uppermost part of these basins, that is referred to ‘recent’ deformation in this paper, can be identified not only along major thrusts but also between major thrusts and the lower trench slope, suggesting these are related to recently active faulting. Several but the largest number of such deformation are distributed along a major thrust located in the middle of the forearc high region, whereas few are done along other major thrusts.

Segmentation and volcano-tectonic characteristics along the SW African continental margin, South Atlantic, as derived from multichannel seismic and potential field data

Regional seismic reflection and potential field data document the South Atlantic's break-up history, between 39°S and 19°S, from the Early Cretaceous onwards. Previous maps of distribution of volcanics along the margin showed volcanics along the whole African margin based on extrapolation of data. Based on previously unpublished marine geophysical data, we found the southernmost 460 km long margin segment to be lacking huge volumes of break-up related volcanic effusives. Northwards, break-up was accompanied by the emplacement of huge volumes of volcanic material, prominently featured in seismic sections as huge wedge-shaped seaward dipping reflectors (SDRs). Detailed mapping of offsets (left- and right-stepping) and variations in structural character of the volcanics reveal the segmentation along and the break-up history of the margin. Several superimposed SDR sequences, suggesting episodicity of volcanic emplacement (divided by periods of erosion and sedimentation), are distinct along southerly lines, losing prominence northwards.A main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non-volcanic to a volcanic margin occurs abruptly.We define four distinct First-order Segments along the 2400 km section of the southwestern African margin covered by our seismic data. From south to north these First-order Segments are: Magma-poor Segment I; Segment II with enormous SDRs volumes; decreasing SDRs volumes in Segment III; Segment IV again with enormous volcanic output, likely influenced by Walvis Ridge volcanism.Most important is that there is no systematic increase in the volumes of the effusives towards the Tristan da Cunha hot-spot. Rather there is an alternating pattern in the SDRs' volumes and widths.The boundary between the volcanic and magma-poor margin segments in the southernmost study area is sharp (10s of km), which we propose is reflected in magnetic anomaly data as well. We suggest that this variability along the margin is mainly due to a change in stretching/rifting character from oblique during the early stages of breakup to conventional seafloor spreading from Chron M4 (∼130 Ma) onwards.

Control of preexisting faults and near-surface diapirs on geometry and kinematics of fold-and-thrust belts (Internal Prebetic, Eastern Betic Cordillera)

We have determined, for the first time, the 3D geometry of a sector of the eastern Internal Prebetic comprised between Parcent and Altea diapirs, combining structural, borehole and multichannel seismic reflection data. The tectonic structure of the Jurassic-Cretaceous carbonate series is characterized by regional ENE-WSW fold-and-thrusts that interact with oblique N-S and WNW-ESE folds, detached over Triassic evaporites and clays. The structural style comprises box-shape anticlines, and N-vergent anticlines with vertical to overturned limbs frequently bordered by reverse and strike-slip faults. The anticlines surround a triangular broad synclinal structure, the Tárbena basin, filled by a late Oligocene to Tortonian sedimentary sequence that recorded folding and thrusting history. The location and geometrical characteristics of fold-and-thrusts may be controlled by the positive inversion of pre-existing Mesozoic normal faults, and by the position and shape of near-surface diapirs composed of Triassic rocks. Therefore, we propose an initial near-surface diapir emplacement of Triassic evaporitic rocks driven by late Jurassic to early Cretaceous rifting of the southern Iberian paleomargin. Thrusting and folding started during the latest Oligocene (∼28–23Ma) roughly orthogonal to the NW-directed shortening. Deformation migrated to the south during Aquitanian (∼23–20Ma), when tectonic inversion implied the left-lateral transpressive reactivation of N-S striking former normal faults and right-lateral/reverse reactivation of inherited WNW-ESE faults. We show two mechanisms driving the extrusion of the diapirs during contraction: lateral migration of a pre-existing near-surface diapir associated with dextral transpression; and squeezing of a previous near-surface diapir at the front of an anticline. Our study underlines the value of 3D geological modeling to characterize geometry and kinematics of complex fold-and-thrust belts influenced by preexisting faults and near-surface diapirs.

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

AbstractWe 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.

Tectonic uplift mechanism of the Goodenough and Fergusson Island gneiss domes, eastern Papua New Guinea: Constraints from seismic reflection and well data

The D'Entrecasteaux Island (DEI) gneiss domes are fault‐bounded domes with ∼2.5 km of relief exposing ultrahigh‐pressure (UHP) and high‐pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene‐Miocene arc‐continent collision and subduction zone subject to Late Miocene to Recent continental extension. To study the style of continental extension accompanying exhumation of the DEI gneiss domes, a grid of 1518 km of 2‐D multichannel seismic (MCS) reflection data and well data is interpreted from the offshore areas surrounding the DEI, including the Trobriand basin and the Goodenough basin. The offshore study is combined with onshore geologic information to constrain the Oligocene to Recent tectonic evolution of the basins. MCS and well data are consistent with the Trobriand basin forming as a forearc basin caused by southward Miocene subduction at the Trobriand trench. At ∼8 Ma, the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1–2.5 km with few normal faults deforming the basin fill. South of the DEI, the Goodenough rift basin developed after extension began (∼8 Ma) with the hanging wall of the north‐dipping Owen‐Stanley normal fault bounding the southern margin of the basin. The lack of evidence of upper crustal extension accompanying subsidence in the Trobriand and Goodenough basins suggests depth‐dependent lithospheric extension from 8 to 0 Ma has accompanied uplift of the DEI gneiss domes and supports schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, postorogenic lower crust, far‐field extension from slab rollback, and an inverted two‐layer crustal density structure.