Multichannel Seismic Reflection Data Research Articles

Mapping normal faults on the outer slope of the western Kuril Trench based on recent seismic reflection and bathymetric data

To obtain novel information regarding normal faults that could cause tsunamis and large earthquakes on the outer slopes of the Kuril Trench, recently acquired multichannel seismic reflection (MCS) and bathymetric data are processed in this study to interpret the normal faults on the outer slope of the western Kuril Trench from near the Erimo Seamount to near the Nosappu Fracture Zone. This work also assesses the distribution and characteristics of the faults through comparisons with previous studies on the outer slope faults of the Japan Trench. The results of this study show that faulting starts approximately 100 km seaward of the trench axis, and a comparison with the normal fault distributions on the outer slope of the Japan Trench presented in previous studies reveals that the outer slope region of the Kuril Trench is narrower and more densely faulted than that of the Japan Trench. Additionally, the seismic and bathymetric data show that the Erimo and Takuyo-Daiichi Seamounts are deformed by normal faults. However, the number of faults in the seamount areas is lower than that in areas without them, which also affects the scale of the fault throws. The results of the MCS survey reveal that the thickness of the oceanic crust varies between 6 and 10 km due to the presence of these seamounts. Considering the positions of the outer swells presented in previous studies, the results suggest that the crustal and plate structures in the study area influence the distribution and formation of faults on the outer slope of the Kuril Trench.Graphical

Clustered slumping in the northern South China Sea: Implications for chronology and geohazards

Seismic facies analysis is the most widely used method to identify event deposits from subaqueous environments. However, the internal structures of a chaotic or transparent seismic unit that represent event deposits are usually poorly imaged. This is primarily due to the limited resolution (usually <10 m) of commonly available multi-channel seismic reflection data. As a consequence, potential (sub)meter-thick, interbedded background sediments that may divide the chaotic layer cannot be discerned on such a seismic profile. The result of this, is that a complex of multiple moderate-thickness event layers can be misinterpreted as a single, thick event layer and this can greatly impact age-depth model reconstruction and geohazard assessment. One approach to resolve the problem is to correlate seismic data with high-resolution sediment core analysis. To address the problem in the South China Sea, we combine multiple methods to identify event deposits in the IODP holes U1499A and U1432C. Our dataset reveals that: (1) the previously interpreted ~50 m-thick slumping unit in the region is a complex of multiple moderately sized units; (2) the slumping events are clustered between 0.6 and 0.4 Ma. Using our new understanding of event emplacement, we define event-free age models for mass wasting on the margin of the South China Sea, improving our understanding of local geohazards. Our methods here represent a sedimentological approach which could be used in other subaqueous environments to reconstruct event-free age models and geohazard histories.

Post-Caledonian tectonic evolution of the Precambrian and Paleozoic platform boundary zone offshore Poland based on the new and vintage multi-channel reflection seismic data

Post-Caledonian tectonic evolution of the Precambrian and Paleozoic platform boundary zone offshore Poland based on the new and vintage multi-channel reflection seismic data

East Siberian Sea Composite Tectono-Sedimentary Element, Siberian Arctic

The East Siberian Sea Composite Tectono-Sedimentary Element (ESS CTSE) occupies most of the central and southern parts of the East Siberian Sea (ESS) and extends onshore into the Chukchi Peninsula. The first multichannel seismic reflection data, acquired in this region in 1989 and 1991 revealed its structural complexity and the presence of a deep sedimentary depocentre in its central part. In the 2010s, the first long-offset seismic data provided a much better understanding of the geology of this remote segment of the Arctic shelf. It includes a large extensional basin that started to form in the Albian (?)-Cenomanian, postdating the Barremian-Aptian Chukotkan orogeny and formation of the New Siberian-Chukchi Fold-and-Thrust Belt (FTB). The latter experienced extensional collapse and developed into the tectonic basement that underlies the CTSE over its entire extent. The ESS CTSE is mainly composed of Upper Cretaceous-Holocene siliciclastic sedimentary strata with a total thickness up to 14 km. This succession has not been drilled deep enough to test its hydrocarbon potential since the beginning of exploration around 2013. However, the strata contain abundant terrestrial organic matter and are considered as potential source rocks sufficient for generating gas accumulations.

Eurasian Arctic Rifted Margin Composite Tectono-Sedimentary Element

The Eurasian Arctic Rifted Margin (EARM) spans a poorly studied transition region between the Eurasian continental shelf and the deep-water Nansen Basin. It is a conjugate margin to the Lomonosov Ridge, as both domains were united before the opening of the oceanic Eurasia Basin. The EARM is considered as a Composite Tectono-Sedimentary Element (CTSE) consisting of two individual tectono-sedimentary elements (TSEs): (1) an undefined group of poorly known pre-rift Mesozoic and Paleozoic sedimentary successions; and (2) Late Cretaceous–Paleocene synrift strata. The latter are bounded by a c. 56 Ma break-up unconformity and are overlain by the Eurasia Basin prograded margin TSE, which is not included in EARM. In the absence of drilled wells, we utilize multichannel seismic reflection (MCS) data for seismic stratigraphic correlations with nearby, better-explored, areas. The EARM CTSE is subdivided into three along-strike segments (from west to east): Svalbard–Barents Sea, the North Kara Sea and the Taimyr–Laptev Sea. Across-strike, the EARM CTSE is composed of a proximal zone of stretched crust characterized by normal faulting and a relatively narrow distal transition zone that may contain unroofed serpentinized mantle. The lack of drilled wells makes it challenging to evaluate the petroleum potential of the CTSE, although some indications of a working petroleum system presence exist.

Seismic oceanography of internal solitary waves offshore the South Island, New Zealand

Seismic oceanography has been widely used in the study of internal solitary waves (ISWs) in recent years, and has achieved remarkable results. In this paper, we analyzed the multi-channel seismic reflection data in the Canterbury Basin offshore New Zealand from January 9 to January 29, 2000, collected by R/V Maurice Ewing. We observed 4 groups of ISWs (labeled ISW1s to ISW4s) on 4 seismic survey lines. We studied their waveforms and propagation speeds in detail. There are two theoretical structures used to describe the vertical waveform of ISWs: the first-order nonlinear vertical structure and the linear vertical structure. We found that ISW1s fit the nonlinear structure well, ISW3s and ISW4s fit the linear structure, and ISW2 does not fit either one. As the water depth increases, the waveforms of all ISWs gradually widen. Two satellite SAR images reveal that all ISWs generally travel shoreward across the isobaths. However, the propagation direction of ISW1s is about 354°-360° (clockwise from due north), different from the propagation directions of other ISWs (about 22°-26°), which explains why ISW1s have the largest characteristic half-height width. The estimated propagation speeds are close to the theoretical speeds, confirming our speed correction method. In the end, we also discuss the interaction of ISWs and eddies.

Characteristics and migration of subaqueous sand dunes influenced by internal solitary waves in the Dongsha Region, Northern South China Sea

Seabed data acquired from the Dongsha Region in the northern South China Sea (SCS) revealed the detailed morphology of sand dune fields on the upper continental slope, at depths between 230 and 1240 m. High-resolution multibeam echosounder bathymetry and multichannel seismic reflection data were used to describe and interpret the distribution, characteristics, and migration of these sand dunes. The sand dunes were distributed as patches on a generally gentle continental slope (gradients≤0.58°), on a seafloor mainly covered by silty and coarse sand. The dunes exhibited distinctly different distribution patterns, geometries, and migration behavior. The trough-to-trough wavelength ranged from 47 to 555 m, the height was 0.1 to 19.8 m, the crest length was 100 m to 10s of km, and the dominant orientation was NE-SW. Based on their distribution and characteristics, the sand dunes were further categorized into (i) sinuous, (ii) barchan, (iii) reticular, (iv) restricted, and (v) superimposed types. Most sand dunes migrated to both NW and SE directions. We infer that, in the Dongsha Region, northern SCS, sand dunes formed and migrated due the world's largest observed internal solitary waves (ISWs). Wave-wave and wave-topography interactions occur as ISWs propagate and undergo polarity conversion during shoaling process, reflection, refraction, diffraction at the Dongsha Atoll. These processes bring about variable and complex local hydrodynamic conditions in the Dongsha Region, dissipate vast amounts of energy, erode the seafloor, and supply the sediment and energy required to sustain the formation and migration of very large sand dunes. Our findings not only enhance our understanding of the intricate relationships between topographic and geomorphologic evolution, sedimentary dynamics and mesoscale motions, but also hold significant importance for various hydrographic applications, such as navigation and submarine pipeline installation.

East Barents Basin and Admiralty High Composite Tectono-Sedimentary Elements, Barents Sea

Abstract The East Barents Sea (E Barents Sea) includes two major tectonic elements, the East Barents Basin (EB Basin) and the Admiralty High, which are, in the context of the volume, characterized as composite tectono-sedimentary elements. Whereas they share some stratigraphy, their early history may differ. The EB Basin is a dominant structural feature of the Barents Shelf, and one of the largest and deepest sedimentary basins in the Arctic. Admiralty High rims the EB Basin in the east. The advance in understanding the E Barents Sea geology was achieved during 1980s and 1990s mainly due to a significant amount of multichannel reflection seismic data and a number of drilled deep exploration wells. These wells tested many large anticlinal structures and made several large gas discoveries including a giant Shtokman gas and gas condensate accumulation. In the following decades, many important geological results were published in the international and Russian literature. In this chapter we provide a summary of the geology and petroleum geology of the E Barents Sea based on published results and public-domain geological reports.

Fault structure, seismicity, and magmatism of the Littoral Fault zone, northern South China sea: Insights from high-resolution seismic reflection data

Four destructive M7+ earthquakes and 18 M6+ earthquakes demonstrate that the Littoral Fault Zone (LFZ) is one of the most seismically active regions in the offshore area of South China. However, the location, geometry, and structural characteristics of the LFZ, which spans over 1000 km, have not been properly examined. In this study, we investigate the precise position of the LFZ offshore East Guangdong (EGD) and map the fault structure using recently acquired high-resolution multi-channel seismic (MCS) reflection data. The LFZ consists of an array of normal faults that collectively form a terraced sidewall fault zone linked to the continental rifting of the South China Sea (SCS). This fault zone is approximately 35–75 km wide and runs parallel to the shoreline. The primary faults of the LFZ EGD segment correlate to a high-gravity gradient zone and serve as the northern boundary of the Zhu-I Depression. During the syn-rift phase, the LFZ experienced significant intra-plate deformation, resulting in fault throws of up to 1000 m. This deformation was locally influenced by pre-existing structures, giving rise to a listric geometry at depth. In the post-rift period, the LFZ exhibited inherited activity along its rift-related faults. The presence of growth fault structures suggests that the LFZ remained active throughout the Cenozoic, with numerous active faults extending practically to the seafloor. The negative flower structure confirms the strike-slip character of the LFZ, which is influenced by the modern tectonic stress field. Intense multi-phase Cenozoic magmatism, including igneous intrusion, volcanism, and lava flows, has been detected in the offshore area of Nan'ao Island. The frequent earthquakes in the study area are primarily attributed to far-field stress release resulting from the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Additionally, the intense magmatism weakens crustal structures, further facilitating stress release and the frequent occurrence of earthquakes.

The ocean-continent transition in the Western Central Red Sea

The Red Sea is an important example of a continental rift transitioning slowly to an oceanic basin. However, structures that can inform us of how that transition occurred have been poorly reported because deep seismic reflection data capable of imaging basement under the rift sediments are generally lacking publicly. Three lines of multichannel seismic reflection data have recently been published revealing structures on the Nubian side of the central part of the basin. In this study, we reassess these data in the light of recent studies of the central Red Sea. Over continental crust, the data reveal reflection sequences likely due to strata at or near the base of the evaporites, in two cases with varied dips suggesting the presence of syn-rift growth stratigraphy. Almost all of those reflections dip downwards towards the rift axis, not away as would be expected from tilted fault blocks of bookshelf faulting types. That observation, and low relief of basement, confirm inferences made earlier based on gravity anomalies that this part of the Red Sea lacks large-relief fault escarpments and is most likely a syn-rift sag basin. In the transition to oceanic crust, an abnormally broad magnetic anomaly of estimated Chron 5 age is found not to be associated with structures such as sills, so it likely arises from deeper sources. One of the seismic lines traverses a ridge in Bouguer gravity anomalies that runs across the axis. This feature has previously been interpreted as a volcanic ridge similar to those observed at other ultra-slow spreading ridges. The seismic data reveal diffuse basement reflections and confirm that the record immediately above basement lacks reflections typical of sedimentary strata. Both observations are consistent with the presence of oceanic crust. Modelling of gravity anomalies suggests the ridge is likely underlain by igneous intrusive rocks displacing mantle rocks, as expected for a volcanic ridge. The seismic data, combined with recently updated multibeam and high-resolution sparker seismic results, further suggest how the evaporite movements have been modulated by basement topography. These results add to our knowledge of the evaporite movements and continent-ocean transition structures in the central Red Sea.

Developmental history of a newly discovered chain of paired mound and depression structures on the southwestern continental shelf of the Ulleung Basin, East Sea, Korea

A chain of paired mound and depression structures are presented in the southwestern continental shelf of the Ulleung Basin, East Sea, Korea. We integrate multibeam bathymetry to identify topographic features, sub-bottom profile and multi-channel seismic reflection data to confirm geological features, and piston cores to understand the sediments of the mounds and depressions. Paired structures occur with a northwest–southeast orientation at water depths between 130 and 150 m and have similar scales of 10–20 m in height and depth. The mounds consist of cold-water corals and authigenic carbonate with shelly sand, whereas, the surficial lithology of depressions are only composed of siliciclastic deposits. The mounds appeared to have grown from the depression based on the observation for morphological features in the cross-section profiles of the paired mound and depression structures. Seismic profiles show acoustic blanking anomalies and seismic chimneys, indicating that the topographic relief formed under the influence of gas seepage. The petrological and geochemical signature of carbonate chunks also displayed gas leak effects different from CWC. This study suggests a developmental history model with paired mound and depression structures, in which gas/fluid escape causes depressions and the selective coral growth creates mounds growing on one side of these depressions. The mounds had sufficiently grown, depressions would have been further developed in interaction with the bottom current.

Enhancing earthquake detection from distributed acoustic sensing data by coherency measure and moving-rank-reduction filtering

Distributed acoustic sensing (DAS) enables the recording of earthquake signals at an unprecedented low cost with high durability using fiber optic cables. It is often compromised by relatively lower data quality in single-channel measurements compared with traditional seismic receivers but compensated by high-density recordings from hundreds of channels per earthquake event. The multichannel nature of DAS data sets facilitates the applications of well-developed coherency-based denoising methods arising from reflection seismology for detecting more earthquakes. We first introduce a coherency measure for detecting earthquake signals from DAS data sets. Then, we apply a moving-rank-reduction (MRR) filter to enhance the DAS data quality so as to improve the earthquake detectability. The MRR filter is tailored from a rank-reduction filter that is widely used in processing multichannel reflection seismic data. We find that a simple band-pass or median filter is incapable of revealing weak signals generated from small-magnitude or far-away earthquake events, whereas the MRR filter significantly improves the signal-to-noise ratio that enables the detection of those weak signals. We apply the MRR method and the coherency measure on the San Andreas Fault Observatory at Depth DAS data sets for denoising and earthquake detection. As a result, our framework detects all 31 cataloged events, outperforming the previous detection of 25 events using the same data set.

Seismically-derived porosity of deep-sea sediments over the last 74 Ma in the equatorial Atlantic Ocean: Implications for paleo-climate

Deep-sea sediments record the geological and climate history of the Earth. The seismic properties of these sediments, namely the P- and S-wave velocities, depend on their chemical compositions, grain size, age and depth of burial, which, in turn, influence the porosity. We use multichannel seismic reflection data to estimate the sediment thickness and P-wave velocity over 4-74 Ma old oceanic crust on the African Plate and 3-27 Ma old crust on the South American Plate in the equatorial Atlantic Region. We then use the P-to-S converted waves from the basement-sediment interface recorded by 69 multi-component ocean bottom seismometers to determine the P- and S-wave velocity ratios (VPVS) over 3 to 74 Ma. We find that the VPVS decreases from 8.9 ± 2.0 at 3 Ma to 5.0 ± 1.0 at 17 Ma but abruptly increases to 19.5 ± 2.8 at 37.5 Ma. Then, it gradually decreases for crustal ages >37 Ma, reaching 7.0 ± 0.3 at 74 Ma. These variations are almost symmetric across the Mid-Atlantic Ridge over 4-27 Ma, suggesting a relation to climate change rather than regional effects. We estimate the average sediment porosity using the VPVS and find that the average porosity decreases from 89.2 ± 1.5% at 37.5 Ma to 69.9 ± 1.2% at 74 Ma, possibly associated with accumulation of terrigenous input from Africa. Between 4 and 37 Ma, the porosity fluctuates between 70% and 85%. Such a fluctuation correlates well with the decrease in the global atmospheric CO2 from 800 ppm to 400 ppm during the Oligocene and with carbonate compensation depth, which are possibly related to fauna/flora diversification and calcareous sediment accumulation. The variation in porosity at 23 and 17 Ma also appears to be linked with major climate shifts, suggesting that sediment porosity could be used to shed light on the past climate and fauna/flora characterization.

Integration of recent drill results with multichannel seismic reflection data: New inferences on the subsurface configuration and sedimentary history of the northwestern Bay of Bengal

The study pertains to the use of two industry wells and one IODP drill site in conjunction with high quality multichannel seismic reflection data in the northwestern Bay of Bengal. The industry wells provide subsurface information from Early Cretaceous period while the IODP drill site U1444 gives information about deposition younger to Miocene. Their locations over the seismic profiles facilitated the assignment of ages to the prominent seismic horizons thereby yielding a revised seismic stratigraphy of the northwestern Bay of Bengal.The ages of the oldest seismic horizons are in agreement with the recently published ages of the underlying oceanic crust. The sediments up to Late Cretaceous time are thick west of 85°E Ridge, while to its east, they are thin. The sediments up to Paleocene time thin towards offshore, implying that they are land derived (from East Coast of India). Fan sedimentation commenced in the Eocene time and was excessive since Miocene. The transparent sequence between the Late Pleistocene and Pliocene unconformities is thin over the 85°E Ridge. The western extents of the Quaternary subfans above the Late Pleistocene unconformity are demarcated in the study area.Further, the study suggests that the emplacement of the 85°E Ridge took place in the Middle Cretaceous period. Faulting/folding at the continental rise occurred around Miocene time and maybe linked to intraplate deformation that occurred in the Central Indian Ocean Basin. Thus, the inferences drawn from this study help to provide a reliable sedimentary history of the northwestern Bay of Bengal forming an updated base for future investigations.

Shallow structure of the Northern Chilean marine forearc between 19°S - 21°S using multichannel seismic reflection and refraction data

Seismic investigations and scientific ocean drilling projects have documented a remarkable degree of geologic heterogeneity in marine forearc basins in convergent margins worldwide. The mechanisms for offshore basin formation and evolution are, however, often not completely understood, especially for erosive margins. This study integrates high-resolution bathymetry, seismic reflection lines, and refraction-based P-wave velocities to better understand the shallow structure of the northern Chilean erosive convergent margin between 19° and 21°S. The data resolve a regionally developed forearc basin, which contains the previously recognized upper slope-shelf Iquique Basin, and which reaches a maximum depth along the middle slope, containing up to 2 km of accumulated sediments. The basin geometry appears to be tectonically controlled by major north-south oriented crustal faults, which (to first order) govern the morphology and shallow structure of the marine forearc. We propose subsidence due to enhanced subduction erosion by high relief/seamount subduction as a mechanism for basin formation. Spatial correlation between the forearc basin and the highest slip area for the Iquique earthquake highlight the relevance of future studies to understand the relationship between the shallow structure of the marine forearc and the processes occurring on the subduction interface.

Tunnel valleys in the southeastern North Sea: more data, more complexity

Abstract. Large Pleistocene ice sheets have produced glacial structures both at and below the surface in northern Europe. Some of the largest and most erosive structures are so-called tunnel valleys (TVs): large and deep channels (typically up to 5 km wide and up to 400 m deep, with lengths up to 100 km), which formed below ice sheets. Although the subject of many studies, the details of their formation and fill are still not well understood. Here, we present an update on the distribution of TVs in the southeastern North Sea between Amrum and Heligoland based on a very dense grid of high-resolution 2D multi-channel reflection seismic data (400 m line spacing). The known tunnel valleys (TV1–TV3) in that area can now be traced in greater detail and further westwards, which results in an increased resolution and coverage of their distribution. Additionally, we were able to identify an even deeper and older tunnel valley, TV0, whose orientation parallels the thrust direction of the Heligoland Glacitectonic Complex (HGC). This observation implies a formation of TV0 before the HGC during an early-Elsterian or pre-Elsterian ice advance. For the first time, we acquired high-resolution longitudinal seismic profiles following the thalweg of known TVs. These longitudinal profiles offer clear indications of an incision during high-pressure bank-full conditions. The fill indicates sedimentation in an early high-energy environment for the lower part and a subsequent low-energy environment for the upper part. Our results demonstrate that a very dense profile spacing is required to decipher the complex incisions of TVs during multiple ice advances in a specific region. We also demonstrate that the time- and cost-effective acquisition of high-resolution 2D reflection seismic data holds the potential to further our understanding of the incision and filling mechanisms as well as of the distribution, complexity and incision depths of TVs in different geological settings.

Factors for pre-conditioning and post-failure behaviour of submarine landslides in the margins of Ulleung Basin, East Sea (Japan Sea)

Glide planes, the basal surface or failure surface upon which submarine landslides initiate, commonly develop along weak, distinctive stratigraphic horizons but their lithological/mechanical characteristics and genetic mechanisms remain largely unknown. We use 2-D multi-channel seismic reflection data, integrated with multibeam bathymetry and deep drilling data from the Ulleung Basin margins, East (Japan) Sea, to: (1) identify and characterize the nature of glide planes associated with submarine landslides; (2) understand the influence of climate-modulated factors in preconditioning slope failures; and (3) document the post-failure evolution of the landslides. 24 glide planes were identified among 38 submarine slides (SL1 – SL38), which correspond to regionally continuous, positive-polarity high-amplitude seismic reflections. Well-seismic integration support ca. 340 ka – 1,200 ka ages of formation of the major glide planes in the southwestern and western margins of the basin. These glide planes developed at the interface between clay-rich sediment deposited during glacial periods and biogenic diatom-rich sediments deposited during interglacial periods. Physical, mineralogical and geochemical properties determined by density, porosity, gamma-ray, shear strength, X-ray diffraction, and X-ray fluorescence data reveal significant lithological and mechanical changes at the interface between these two lithologies. We therefore infer that these interfaces dictate the position of failure surfaces, with the diatom-rich layers acting as a weak layer. Excess pore pressure in these layers is likely due to initial high-water contents (up to 75%) and high compressibility; this is considered an important pre-condition for failure. In contrast, the glide planes along the northwestern margin of the Ulleung Basin (SL34 – 37) are older (ca. 1,200 ka – 2,140 ka). Seismic data further reveal three distinct contrasting styles of landslide post-failure behavior throughout the margins: (1) evacuated slide scars with areas of smooth seafloor; (2) slide scars with residual debris consisting of blocky sediments; and (3) slide scars with buried intact sediment blocks in front of the headwalls. Lateral variability of fluid flow, sediment composition, and mechanical properties of basal ‘weak’ layer(s), or the magnitude of earthquakes may have contributed to forming different types of mass-transport deposits (MTDs). Overall, these results show that landslide formation in the East (Japan) Sea result from a complex climatic, volcanic and tectonic interplay that controlled the formation of weak layers. Some of these layers extend regionally and can be identified and mapped by remote geophysical methods and targeted drilling.

Crustal structure and magmatism of the Marvin Spur and northern Alpha Ridge, Arctic Ocean

SUMMARYThe Marvin Spur is a 450-km-long east–west trending escarpment along the northernmost periphery of the Alpha Ridge, starting about 500 km from the coasts of Ellesmere Island and Greenland off the Arctic Ocean margin of North America and running subparallel to the Amerasian margin of the continental Lomonosov Ridge. This region was investigated as part of the Canada–Sweden Polar Expedition in 2016, from which two seismic profiles are presented. The first is a 165-km-long line along the crest of the Marvin Spur. The second is a 221-km-long line extending southwestward from the spur to the northern flank of the Alpha Ridge within the Cretaceous High Arctic Large Igneous Province (HALIP). Multichannel seismic reflection data were acquired along both lines using a 100-m-long streamer, and the airgun shots were also recorded using 16 sonobuoys and 5 stations on the sea ice to calculate a velocity model for the crust from forward modelling of seismic traveltimes. The Marvin Spur profile reveals up to 1100 m of sedimentary rocks on top of a 1-km-thick series of basalts (4.5–5.1 km s−1). Upper and lower crust have velocities of 5.8–5.9 km s−1 and 6.2–6.3 km s−1, respectively, with the upper crust being 1–2 km thick compared to around 13 km for the lower crust. A wide-angle double seismic reflection manifests the top and base of a 6-km-thick lower crustal layer that we interpret as magmatic underplating beneath the continental crust of the Marvin Spur. We correlate a high-amplitude magnetic anomaly on Marvin Spur with a comparable anomaly on Lomonosov Ridge by invoking 110 km of dextral strike-slip motion. Assuming that HALIP-related magmatic deposits generate these anomalies, the strike-slip motion pre-dates the main phase of magmatism (latest Cretaceous, 78 Ma). On the northern Alpha Ridge, sediments are around 1-km-thick and cover a 700 to 1700-m-thick series of basalts with velocities of 4.4–4.8 km s−1. Below is a 3-km-thick layer with intermediate velocities of 5.6 km s−1 and a lower crust with a velocity of 6.8 km s−1. Moho depth is not resolved seismically, but gravity modelling indicates a total thickness of 13 or 18 km for the igneous crust except for the Fedotov Seamount where Moho deepens by about 5 km. Construction of the seamount occurred in multiple magmatic phases, including flow eruptions during deposition of the Cenozoic sedimentary succession post-dating the main HALIP magmatism.

Morpho-sedimentary structure of new mud volcanoes on the Moroccan Atlantic continental margin (Gulf of Cadiz)

Multibeam bathymetry, sub-bottom parametric profiler, multichannel seismic reflection data and sediment cores were used to detail the nature, morpho-sedimentary and internal structure of five newly discovered submarine mud volcanoes (MVs) in the Moroccan margin of the Gulf of Cadiz. The Henriet and Subvent MVs are located at 300–400 m water depth, while Chueca, Demetrio de Armas and Puerto Real MVs are at 1100–1800 m water depth. Two main types of morphologies are identified: regular cone-shaped edifices (Subvent MV) with a pronounced crater (Henriet MV) in the eastern province; and ridge-attached oval-shaped conical edifices (Chueca, Demetrio de Armas and Puerto Real MVs) in the western province. The overall seismic architecture of these MVs is the result of successive events of mud extrusion and outbuilding alternating with periods of dormancy. The Henriet and Subvent MV system is composed of stacked bicones and intrusive complexes, which penetrated upper Miocene-Quaternary sedimentary units rooted in the Gulf of Cadiz wedge. Major phases of mud extrusion and outbuilding took place since the Late Pliocene with re-activation during mid-Pleistocene. Mud breccias interbedded with hemipelagic/contourite sediments were collected for all MVs. Cores attest to recent periods of mud outflows lasting from the Late Pleistocene (180 ky) whereas the end of MV extrusion could date back to historic times (the last 0.6 ky), giving rise to the onset of a new quiescent activity. The most active MV points out to the Subvent MV. These new MVs are formed in response to the extensional and compressional system within the Gibraltar Arc. In the eastern side, MVs are related to extensional faults forming deep sedimentary basins and forcing overpressured fluids to migrate upwards coeval with thick contourite deposits. On the western side, MVs are related to compressional ridges at the front of fold-thrust systems which act, as pathways to deep-seated fluids to ascent to seafloor.

Investigating Eddies From Coincident Seismic and Hydrographic Measurements in the Chukchi Borderlands, the Western Arctic Ocean

AbstractHalocline eddies transport mass and energy across the Arctic Ocean. Seismic oceanography uses multichannel seismic reflection (MCS) data to create high resolution images of the water column, revealing oceanic fine structures. In this paper, we present water column images processed from MCS data acquired during cruise MGL1112 on the Chukchi Borderlands in the western Arctic Ocean. Combined with along‐track images of current velocities measured during MCS acquisition by a hull‐mounted acoustic Doppler current profilers, a total of 23 mesoscale eddies were detected, of which 19 are anticyclonic, and 4 are cyclonic. They correspond to the lentoid and mounded reflections on the seismic images, respectively. These shallow eddies are constrained by the halocline and occur in regions with rugged seafloor. The geometric parameters of these eddies were estimated from the underway data collected during MGL1112. These parameters could be valuable for modeling 3D eddy structures and validating high‐resolution climate projection models. Expendable Bathythermography (XBT) profiles of water temperature and sound speed versus depth were collected during this ruise. Three of the 24 XBT stations sampled eddies opportunely, we found one was warm‐core and two were cold‐core anticyclonic eddies combined with historical conductivity‐temperature‐depth data. The cores of these eddies might be made up of Pacific water. We synthesized the MCS records with coincident and historical hydrographic data which permitted distinctions between the seismic responses of the eddies. Distinctive reflection structures are observed around the eddy core (e.g., chaotic, imbricate, layered, and listric reflections), which could constrain stirring and mixing process of the eddies. These results are useful for better understanding the structure and evolution of the eddies on the Chukchi Borderlands and better understanding regional mass and energy transport processes in the western Arctic Ocean.