Multichannel Seismic Reflection Research Articles

A new 3-D P-wave velocity model for the Gulf of Cadiz and adjacent areas derived from controlled-source seismic data: application to nonlinear probabilistic relocation of moderate earthquakes

SUMMARYThe SW Iberian margin is well known for its complex tectonic setting and crustal structure and by the occurrence of moderate magnitude earthquakes and some great tsunamigenic earthquakes. Fortunately, many seismic reflection and refraction profiles have been carried out, providing detailed information about the crustal structure of the main geologic domains in this region. These studies show a first-order variation due to the transition from oceanic to continental domain, large-scale heterogeneities within the crust and an irregular Moho topography. Routine earthquake locations in this area have been usually computed using a general 1-D velocity model which is clear that cannot account for such a heterogeneous structure. In addition, regional seismic stations used to locate the Gulf of Cadiz seismicity are on land and far away to the east, implying large azimuthal gaps and distances. In this context, a 3-D approach seems necessary to properly solve the crustal velocity field and improve earthquake location in this area. With this purpose, we present a new digital 3-D P-wave velocity distribution for the crust and uppermost mantle derived from previously published controlled-source seismic experiments carried out in SW Iberia and the Gulf of Cadiz over the last 40 yr. We have reviewed more than 50 wide-angle and multichannel seismic reflection and refraction profiles and digitized the most significant published 2-D seismic velocity models, performing an updated compilation of crustal parameters (P-wave velocities and geometry and depth of the main crustal interfaces). These velocities as a function of position and depth have been interpolated using ordinary kriging algorithm to obtain, in the form of a regular georeferenced 20 × 20 × 1 km grid spacing, a high-resolution 3-D P-wave velocity distribution for the crust and uppermost mantle and a continuous Moho depth map of the whole area of this study (33°N–41°N latitude and 15°W–5°W longitude). Since current seismic location tools allow the implementation of 3-D grid structures, we have applied our 3-D model to relocate a selection of moderate earthquakes occurred in the studied region using a probabilistic nonlinear method. In the Gulf of Cadiz area the probabilistic approximation provides maximum likelihood hypocentres located within the uppermost mantle with the majority of depths ranging between 20 and 45 km. This model would subsequently be implemented at the Spanish Seismic Network for the routine relocation of the seismicity of the area.

Magma-hydrothermal interactions at the Costa Rica Rift from data collected in 1994 and 2015

We use co-located CTD/transmissometry casts and multichannel seismic reflection surveys conducted at the Costa Rica Rift (CRR) to provide a better understanding of magma-hydrothermal processes occurring at an intermediate-rate spreading center. Water column observations reveal an ∼200 m thick plume head ∼650 m above the seafloor, which corresponds to a hydrothermal heat output of ∼200 ± 100 MW at the ridge axis. Assuming a hydrothermal vent temperature of 350°C and a discharge area between 104 and 105 m2, this heat output implies a mean crustal permeability within the discharge zone of between 2×10−14 and 6×10−13 m2, and a conductive thermal boundary layer thickness of ∼20 m. The volume of magma required to maintain the current hydrothermal heat output over the past two decades should result in an across-axis axial magma lens (AML) width between 270 and 1300 m, depending on the amount of cooling and crystallization. However, seismic reflection images, acquired in 1994 and 2015, while showing an apparent along-axis growth of the AML from 2.4 to 6.0 km between surveys, also suggest that, as of 2015, the AML has an apparent across-axis width of no more than 300 m, and that magma delivery at the intermediate spreading rate CRR may be episodic on time scales of tens of years. The data on magma-hydrothermal interactions at the CRR collected in 1994 and 2015 suggest that the hydrothermal system may have significantly cooled and crystallized the AML, primarily in the across-axis direction, and that this hydrothermal system may also episodically turn on and off. The current pattern of microseismicity supports this conclusion, with events not only mirroring the AML depth and location beneath the ridge axis, but also having a temporally varying focus.

Influence of Incoming Plate Relief on Overriding Plate Deformation and Earthquake Nucleation: Cocos Ridge Subduction (Costa Rica)

AbstractWe present a 2‐D P wave velocity model and a coincident multichannel seismic reflection profile characterizing the structure of the southern Costa Rica margin and incoming Cocos Ridge. The seismic profiles image the ocean and overriding plates from the trench across the entire offshore margin, including the structures involved in the 2002 Osa earthquake. The overriding plate consists of three domains: Domain I displays thin‐skinned deformation of an imbricate thrust system composed of fractured rocks. Domain II shows ~15‐km‐long landward dipping reflection packages and active deformation of the shelf sediment. Domain III is little fractured and appears to be dominated by elastic deformation, overlain by ~2‐km‐thick landward dipping strata. The velocity structure supports the argument that the bulk of the margin is highly consolidated rock. Thick‐skinned tectonics probably causes the uplift of Domains II and III. The oceanic plate shows crustal thickness variations from ~14 km at the trench (Cocos Ridge) to 6–7 km beneath the shelf. We combine (1) interplate geometry and fracturing degree, (2) tectonic stresses and brittle strain, and (3) earthquake locations, to investigate relationships between structure and earthquake generation. The 2002 Osa sequence nucleated at the leading flank of subducting seamounts in the area of highest tectonic overpressure. Both estimated rock fracturing and modeled brittle strain steadily increase from the leading flank of the subducting seamounts to their top, reflecting the progressive damage caused by the seamount. Therefore, the seismicity and structural‐mechanical evolution of the upper plate reflect the downward propagation of the leading edge of seamounts.

Geologic controls on up-dip and along-strike propagation of slip during subduction zone earthquakes from a high-resolution seismic reflection survey across the northern limit of slip during the 2010 Mw 8.8 Maule earthquake, offshore Chile

Abstract A grid of closely spaced, high-resolution multichannel seismic (MCS) reflection profiles was acquired in May 2012 over the outer accretionary prism up dip from the patch of greatest slip during the 2010 Mw 8.8 Maule earthquake (offshore Chile) to complement a natural-source seismic experiment designed to monitor the post-earthquake response of the outer accretionary prism. We describe the MCS data and discuss the implications for the response of the accretionary prism during the earthquake and for the long-term evolution of the margin. The most notable observation from the seismic reflection survey is a rapid north-to-south shift over a short distance from nearly total frontal accretion of the trench sediments to nearly total underthrusting of undeformed trench sediments that occurs near the northern edge of slip in the 2010 earthquake. Integrating our structural observations with other geological and geophysical observations, we conclude that sediment subduction beneath a shallow décollement is associated with propagation of slip to the trench during great earthquakes in this region. The lack of resolvable compressive deformation in the trench sediment along this segment of the margin indicates that the plate boundary here is very weak, which allowed the outer prism to shift seaward during the earthquake, driven by large slip down dip. The abrupt shift from sediment subduction to frontal accretion indicates a stepdown in the plate boundary fault, similar to the stepovers that commonly arrest slip propagation in strike-slip faults. We do not detect any variation along strike in the thickness or reflective character of the trench sediments adjacent to the change in deformation front structure. This change, however, is correlated with variations in the morphology and structure of the accretionary prism that extend as far as 40 km landward of the deformation front. We speculate that forearc structural heterogeneity is the result of subduction of an anomalously shallow or rough portion of plate that interacted with and deformed the overlying plate and is now deeply buried. This study highlights need for three-dimensional structural images to understand the interaction between geology and slip during subduction zone earthquakes.

The rises of the Amerasia basin, arctic ocean, and possible equivalents in the Atlantic ocean

Main positive morphostructures of the Amerasia Basin, the Lomonosov Ridge, Alpha Ridge, Mendeleev Rise, Chukchi Plateau and Northwind Ridge, have been considered from geomorphological, geological and geophysical aspects. Time and Depth seismic sections have been provided up to the Moho discontinuity from the Lomonosov Ridge and its junction with the Greenland and East-Siberian shelves. Time and Depth seismic sections of the Mendeleev-Alpha rises and Chukchi Plateau are presented too. The sections were set up based on seismic surveys: deep seismic sounding and multichannel seismic reflection. Some similarities have been reflected for the foregoing land structure depth sections and typical sections of the continental crust. Brief geological and geophysical data have been presented for the positive morphostructures of the Atlantic Ocean such as the Rockall and Vring plateaus, the continental nature of which is established beyond all doubt. Genesis of positive morphostructures in the northern Atlantic Ocean and the Arctic Ocean has been connected with processes of continental rifting and concomitant intraplate magmatism.

Rises of the Amerasia Basin, Arctic Ocean, and Possible Equivalents in the Atlantic Ocean

Main positive morphostructures of the Amerasia Basin, the Lomonosov Ridge, Alpha Ridge, Mendeleev Rise, Chukchi Plateau and Northwind Ridge, have been considered from geomorphological, geological and geophysical aspects. Time and Depth seismic sections have been provided up to the Moho discontinuity from the Lomonosov Ridge and its junction with the Greenland and East-Siberian shelves. Time and Depth seismic sections of the Mendeleev-Alpha rises and Chukchi Plateau are presented too. The sections were set up based on seismic surveys: deep seismic sounding and multichannel seismic reflection. Some similarities have been reflected for the foregoing land structure depth sections and typical sections of the continental crust. Brief geological and geophysical data have been presented for the positive morphostructures of the Atlantic Ocean such as the Rockall and Voring plateaus, the continental nature of which is established beyond all doubt. Genesis of positive morphostructures in the northern Atlantic Ocean and the Arctic Ocean has been connected with processes of continental rifting and concomitant intraplate magmatism.

Moho Structure of the Southwest Sub-Basin, South China Sea, from a Multichannel Seismic Reflection Profile NH973-1

Moho structure provides important clues for understanding crustal structure, isostatic state and magmatic flux from mantle to surface. Across-basin Moho structure of the South China Sea (SCS) is important for understanding crustal evolution mechanisms of both continental break-up and seafloor spreading processes. Southwest Sub-basin (SWSB) opened up the latest and has the closest continental margins, making it the best to study the across-basin structure. Multichannel seismic (MCS) reflection data of line NH973-1 that crosses SWSB in NW-SE direction were reprocessed in order to image Moho structure. In MCS data, Moho reflectors are observed in places, which were not revealed in prior researches. The Moho generally shows symmetric structure on both sides of the central rift valley (CRV) and with variations in crustal thickness. Around CRV, the Moho is 2 seconds depth in two-way travel time (TWTT) beneath the igneous basement, which corresponds to 7 km depth, indicating normal oceanic crustal accretion during the ending of seafloor spreading. Close to the continent-ocean boundary (COB), the Moho becomes shallow to 1 second depth in TWTT (3.5 km), implying strong crustal thinning towards the continent, probably because of poor magma supply at the beginning of seafloor spreading. At south COB, the Moho depth under the crust almost reaches zero, which could be explained as a result of exhumed mantle. In addition, two low-angle, deep-penetrating normal faults are observed at south COB. The faults cut across the Moho into the upper mantle, which may be attributed to lithospheric hyper-stretching at COB during continental break-up.

Seismic markers of the Messinian salinity crisis in the deep Ionian Basin

AbstractWe conduct the seismic signal analysis on vintage and recently collected multichannel seismic reflection profiles from the Ionian Basin to characterize the deep basin Messinian evaporites. These evaporites were deposited in deep and marginal Mediterranean sedimentary basins as a consequence of the “salinity crisis” between 5.97 and 5.33 Ma, a basin‐wide oceanographic and ecological crisis whose origin remains poorly understood. The seismic markers of the Messinian evaporites in the deep Mediterranean basins can be divided in two end‐members, one of which is the typical “trilogy” of gypsum and clastics (Lower Unit – LU), halite (Mobile Unit – MU) and upper anhydrite and marl layers (Upper Unit – UU) traced in the Western Mediterranean Basins. The other end‐member is a single MU unit subdivided in seven sub‐units by clastic interlayers located in the Levant Basin. The causes of these different seismic expressions of the Messinian salinity crisis (MSC) appear to be related to a morphological separation between the two basins by the structural regional sill of the Sicily Channel. With the aid of velocity analyses and seismic imaging via prestack migration in time and depth domains, we define for the first time the seismic signature of the Messinian evaporites in the deep Ionian Basin, which differs from the known end‐members. In addition, we identify different evaporitic depositional settings suggesting a laterally discontinuous deposition. With the information gathered we quantify the volume of evaporitic deposits in the deep Ionian Basin as 500,000 km3 ± 10%. This figure allows us to speculate that the total volume of salts in the Mediterranean basin is larger than commonly assumed. Different depositional units in the Ionian Basin suggest that during the MSC it was separated from the Western Mediterranean by physical thresholds, from the Po Plain/Northern Adriatic Basin, and the Levant Basin, likely reflecting different hydrological and climatic conditions. Finally, the evidence of erosional surfaces and V‐shaped valleys at the top of the MSC unit, together with sharp evaporites pinch out on evaporite‐free pre‐Messinian structural highs, suggest an extreme Messinian Stage 3 base level draw down in the Ionian Basin. Such evidence should be carefully evaluated in the light of Messinian and post‐Messinian vertical crustal movements in the area. The results of this study demonstrates the importance of extracting from seismic data the Messinian paleotopography, the paleomorphology and the detailed stratal architecture in the in order to advance in the understanding of the deep basins Messinian depositional environments.

The Esmeraldas Canyon: A Helpful Marker of the Pliocene‐Pleistocene Tectonic Deformation of the North Ecuador‐Southwest Colombia Convergent Margin

AbstractDeciphering the migration pattern of the Esmeraldas submarine Canyon (EC) and its history of cut‐and‐fill allows constraining the Pliocene‐Pleistocene tectonic evolution of the Ecuador‐Colombia convergent margin. Swath bathymetry, multichannel seismic reflection, and chronological data show that the EC is a 143‐km‐long, shelf‐incising, river‐connected canyon that started incising slope apron deposits in the Manglares fore‐arc basin ~5.3 Ma ago. The EC inception appears contemporaneous with the subduction of the Carnegie Ridge that is believed to have initiated 5–6 Myr ago and is considered an indirect cause of the EC formation. During its two‐stage left‐lateral migration, the EC upper‐half scoured deep incisions providing evidences for uplift episodes in the Manglares Basin that are correlated with mid‐Pliocene and Pleistocene regional tectonic events. Glacioeustatic variations contributed significantly to shape the EC and its upslope tributaries by increasing the rate of canyon incision during rapid sea level falls. Faults, folds, and diapirs have structurally controlled the location of the EC and of its tributary canyons, including the Ancon Canyon, which served as the main spillway of the Manglares Basin prior to be cut from its source ~170 kyr ago by the growth of a fault‐related anticline. The margin wedge that hosts the EC is highly unstable as it is cut by active faults and shaken by large subduction earthquakes. Several mass transport deposits have dammed the EC, one of them between >~65 and ~37 kyr causing an impoverishment of detrital material in the trench sedimentation and a possible interruption of the paleoseismological record.

MULTI-CHANNEL MARINE SEISMIC SURVEY AND PRELIMINARY RESULTS OFF-SHORE KARABURUN RIDGE (WESTERN SIGACIK GULF-AEGEAN SEA)

Sıgacık Gulf is located south of Izmir (West Anatolia), the third crowded city of Turkey. The gulf is restricted by two important ridges on land from west to east, Karaburun Ridge and Seferihisar Ridge, respectively. Recent geological and geophysical studies suggested that these ridges are dominated by active strike-slip faults both on land and off-shore. This manuscript presents the results of the high resolution multi-channel seismic reflection survey which carried out onboard K. Piri Reis, research vessel of Dokuz Eylül University, off-shore Karaburun Ridge (Aegean Sea) and surroundings in August-2005. Two main seismic units, lower unit and upper unit, were determined on seismic sections besides the active faults, buried faults and buried folds.

Marine controlled-source electromagnetics with geothermal purposes; central Gulf of California, Mexico

We acquired and interpreted data from two controlled-source electromagnetic profiles, in the Gulf of California off the coast of Santa Rosalía, Baja California Sur, Mexico. This area is interesting from the geothermal point of view due to its proximity to the Guaymas Basin, the La Reforma Caldera, and the Las Tres Vírgenes volcano, the first with intense submarine hydrothermal manifestations and the final one with a geothermal field in production. The survey consisted of the deployment of 10 Ocean Bottom Electromagnetic receivers, distributed along two ~14 km-long profiles. These sensors recorded the two horizontal components of the electric and magnetic fields. The source consisted of a horizontal 200 m electric dipole towed behind the vessel. The processed data were inverted to estimate the subsurface distribution of electrical resistivity using a finite-element two-dimensional multi-frequency program, resulting in adequate fits between observed and calculated data. To estimate how well the models are constrained by the data, sensitivity tests were applied to the models. The tests were based on the Jacobian matrix and a perturbation analysis of selected features of the model. They show that the model in its central part is well constrained by the data down to a depth of about 2.5 km. With the perturbation approach, 16 resistivity features out of the 19 tested turned out to be well constrained by the data. The estimated resistivity distribution was compared with the information obtained from other geophysical methods (bathymetry, gravity, and 2D multi-channel seismic reflection), resulting in reasonably good correlations between them. For the first profile, we find a remarkable similarity with the current conceptual models proposed in the literature for a volcanic geothermal system, where an altered conductive clay cap overlays another with higher electrical resistivity. The second profile shows agreements with anomalies detected with echo-sounder bathymetry suggesting the existence of hydrothermal manifestations.

New insights into the Cretaceous-Cenozoic tectono-stratigraphic evolution of the southern Lofoten margin, offshore Norway

Abstract The tectono-stratigraphic evolution of the southern Lofoten margin and its immediate transition towards the northern Voring margin has been studied in detail utilizing 2D multi-channel seismic reflection profiles, available wells, in addition to gravity and magnetic data. New and better refined structural elements have been mapped within the study area, including (informally named) the West Rost High Fault Complex, Rost Syncline, and Sandflesa High. Furthermore, four main rift phases have been recognised and refined. Late Jurassic-earliest Cretaceous rifting controlled the initial structuring of the main structural elements. Mid-Cretaceous rifting was responsible for initiation of faulting in the West Rost High Fault Complex, while a composite Late Cretaceous rift phase took place and led to westward propagation of fault activity. Paleocene rifting generated new faults and reactivated several earlier faults, prior to continental breakup and seafloor spreading initiation at the Paleocene-Eocene transition. The Bivrost Lineament, separating the southern Lofoten and northern Voring margins, has exhibited a distinct morphological expression during Cretaceous-Cenozoic and is recognised as a structural “corridor” which segments highs and basins/sub-basins. Furthermore, two dome-shaped features have been observed on the southern Lofoten margin and have probably experienced several phases of growth from Late Cretaceous to Miocene times. The domes are located in close proximity to the outer Voring margin and Bivrost Lineament that are believed to have facilitated the transfer of imposed compressional deformation on the NE Atlantic margins. Finally, a comparison of the study area to the conjugate Northeast Greenland margin provides valuable insights on the margin evolution in a regional and conjugate setting.

Tectonostratigraphic framework and depositional history of the deepwater Ulleung Basin, East Sea/Sea of Japan

AbstractThe Ulleung Basin, East Sea/Japan Sea, is a Neogene back‐arc basin and occupies a tectonically crucial zone under the influence of relative motions between Eurasian, Pacific and Philippine Sea plates. However, the link between tectonics and sedimentation remains poorly understood in the back‐arc Ulleung Basin, as it does in many other back‐arc basins as well, because of a paucity of seismic data and controversy over the tectonic history of the basin. This paper presents an integrated tectonostratigraphic and sedimentary evolution in the deepwater Ulleung Basin using 2D multichannel seismic reflection data. The sedimentary succession within the deepwater Ulleung Basin is divided into four second‐order seismic megasequences (MS1 to MS4). Detailed seismic stratigraphy interpretation of the four megasequences suggests the depositional history of the deepwater Ulleung Basin occurred in four stages, controlled by tectonic movement, volcanism, and sea‐level fluctuations. In Stage 1 (late Oligocene through early Miocene), syn‐rift sediment supplied to the basin was restricted to the southern base‐of‐slope, whereas the northern distal part of the basin was dominated by volcanic sills and lava flows derived from initial rifting‐related volcanism. In Stage 2 (late early Miocene through middle Miocene), volcanic extrusion occurred through post‐rift, chain volcanism in the earliest time, followed by hemipelagic and turbidite sedimentation in a quiescent open marine setting. In Stage 3 (late middle Miocene through late Miocene), compressional activity was predominant throughout the Ulleung Basin, resulting in regional uplift and sub‐aerial erosion/denudation of the southern shelf of the basin, which provided enormous volumes of sediment into the basin through mass transport processes. In Stage 4 (early Pliocene through present), although the degree of tectonic stress decreased significantly, mass movement was still generated by sea‐level fluctuations as well as compressional tectonic movement, resulting in stacked mass transport deposits along the southern basin margin. We propose a new depositional history model for the deepwater Ulleung Basin and provide a window into understanding how tectonic, volcanic and eustatic interactions control sedimentation in back‐arc basins.

The Tagus River delta landslide, off Lisbon, Portugal. Implications for Marine geo-hazards

The stratigraphy of the Tagus river ebb-tidal delta off Lisbon (Portugal) is investigated using high resolution multichannel seismic reflection profiles with the purpose of searching for sedimentary or erosive features associated with landslides. The Tagus delta is sub-divided in two prograding seismic units of 17 ky to 13 ky and 13 ky to Present based on the calibration of seismic lines using gravity and box-cores in the Tagus pro-delta. We report the existence of a buried landslide with 11 km of length, 3.5 km of width and a maximum thickness of 20 m that accounted for the collapse of half of the upper unit of the Tagus river delta front in Holocene times. The non-collapsed half of the delta front contains extensive shallow gas of still unknown origin and nature. An estimated age of ~8 ky BP for the Tagus delta landslide is proposed based on stratigraphic correlation. The trigger mechanisms of the newly identified Tagus landslide are discussed as well as of the several landslides also found in the lower delta unit. These findings present a first step towards a future assessment of the susceptibility of the nearby coastal areas and the off-shore infrastructures to hazards related to such large collapses.

Discovery of numerous pingos and comet-shaped depressions offshore southwestern Taiwan

High-resolution bathymetry collected with an autonomous underwater vehicle (AUV) along the flanks of three ridges of the accretionary prism offshore southwestern (SW) Taiwan revealed more than 650 elongated depressions in water depths ranging from 1155 to 1420 m. The depressions are between 12 and 129 m long, 5 to 70 m wide, and up 9 m deep at their center and shallowing downslope to about 1-m depth. Due to their shape in downslope cross section, they are termed comet-shaped depressions (CSD). The CSD occur in patches of more than 100 with densities of 53 to 98 CSD/km2. In addition, seven topographic mounds were mapped and interpreted as pingos, which remotely operate vehicle (ROV) observations and sampling show to be covered with authigenic carbonate. These features overlie areas where multichannel seismic reflection (MCS) profiles show bottom simulating reflectors (BSR) and dipping strata extending from below the BSR to near the seafloor. We consider comet-shaped depression, a new type of pockmark, forms on a sloping seafloor where fluids expulsion occurred. We also suggest that the two types of distinctive geomorphic features are attributed to fluid venting which occurs at different rates, with the mounds developing slowly over time, but the CSD forming in discrete events perhaps associated with large earthquakes.

Detection of Ocean Internal Tide Source Oscillations on the Slope of Aogashima Island, Japan

AbstractBarotropic tidal currents over bottom topography force density surfaces to oscillate vertically and thereby to act as quasi‐stationary internal tide sources. Deploying a seafloor pressure gauge array with an aperture of 30 km for a year (2014–2015), we detected the low‐mode semidiurnal internal tidal waves propagating with a horizontal phase speed of ~1 m/s in the onshore and offshore directions over the array along the eastern slope of Aogashima Island, south of Japan. The amplitudes of the offshore propagating waves were greater than those of the onshore propagating waves, and both were positively correlated with the amplitudes of the local semidiurnal tide, which peaked in September and March. A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed onshore and offshore internal tidal wave propagation. The model indicated a standing wave region on the slope, where offshore propagating waves interact with standing waves locally pinned to the slope. Along the same profile over a distance of 100 km, we conducted seismic‐oceanographic analysis of the legacy multichannel seismic reflection data to retrieve vertical cross sections of the reflecting layers, which indicated sharp temperature changes in the ocean. Many of the slant reflecting layers were subparallel to the contour lines of the semidiurnal internal‐tide‐associated temperature anomalies in the JCOPE‐T model, suggesting a causal link between the fine reflection layering structure and the semidiurnal low‐mode internal tidal field.

Tethys ocean withdrawal and continental peneplanation—An example from the Galilee, northwestern Arabia

During the early Oligocene northeast Africa (Ethiopia, Sudan, Egypt) and Arabia (Yemen, Saudi Arabia, Jordan, Sinai and southern Israel) progressively emerged from marine conditions. Their complete subaerial exposure was accompanied by widespread erosion. Its products were transported across northeast Gondwana to the nearby Mediterranean and Mesopotamian basins while leaving behind a Regional Truncation Surface (RTS). The uplift mechanism is debated. Some claim it results from the Oligocene updoming of Afar mantle plume across northeastern Gondwana. Others suggest GondwanaEurasia convergence that reached hard-docking during the Oligocene is the dominant process. Miocene-to-Holocene post-truncation tectonics deformed the leveled RTS (peneplain) relief while erosion continued. As a result, only scarce and widely distributed evidence for erosion remained across Arabia. The current study addresses this debate through the examination of the morphological development of the converging plate on the verge of entering the subduction zone. The study focuses on the mode of erosion across the Galilee basins of northern Israel. Based on the interpretation of 69 multichannel seismic reflection profiles (extending along 800 km), correlated with 68 boreholes and all previous geological mapping, we distinguish between pre-, syn- and post-truncation stratigraphic units. Results show that the RTS subcrop comprises a mosaic of diachronous formations, spanning the Upper Cretaceous and Oligocene. Stratigraphy constrains the overall duration of truncation between Late Eocene and Early Miocene. A new observation into the truncation period shown here enhances the morphological expression of ocean bottom exposure. The maximum thickness of strata removed reaches ˜1300 m during a 7 ± 2 million year time window. A conservative time estimation yields a weathering rate of 0.186 ± 0.058 mm per year, under a subtropical climate suggested for the Oligocene in this region. Preservation of syn-truncation sediments in local fault-bounded depressions attests to a regional NNE trending flexural uplift accompanied by local extensions. Truncation sediments derived from these erosional events accumulated across the western and eastern flanks of the Galilee, under marine conditions. We suggest that this flexure developed along a northward propagating arm of the Afar plume. This case study may serve as an example for the morphological development of a tectonic plate before its entrance into a subduction zone.

Morphology and genesis of giant seafloor depressions on the southeastern continental shelf of the Korean Peninsula

We identify and describe five giant seafloor depressions from the southeastern continental shelf of the Korean Peninsula using multibeam bathymetry, sub-bottom profiler, and multi-channel seismic reflection data, supplemented by piston cores. Multibeam bathymetry data from the shelf show four crescent-shaped depressions (SD1 to SD4) and one near-circular depression (SD5) within a group of NW-SE trending depressions, the largest covering an area of about 7 km2 on the seafloor. The depressions reach up to ~4.5 km in width and ~2 km in length and have asymmetric cross-sections. Some have depths as large as 40 m below the surrounding seafloor with walls as steep as 45°. The depressions are confined to water depths between 130 and 170 m and bounded on the north by a large submarine channel that was plausibly formed by fluvial or tidal processes during the Last Glacial Maximum (LGM) sea-level lowstand. Multi-channel seismic and sub-bottom profiler data reveal truncated depression walls and the presence of sediment drift deposits within the depressions, indicating that both erosion and deposition are active processes. Flaser and lenticular bedding in the cored drift deposits along with variable grain size (ranging between ~2.6 phi and ~4.3 phi) are diagnostic features of the bottom currents influenced by tidal forces. Depressions SD1 to SD4 lack evidence of fluid or gas escape. In contrast, many features of depression SD5 are characteristic of gas escapes and blowouts, including acoustic anomalies, a 20-m-high carbonate mound or carbonate-encrusted mound, and mud dikes and mud patches in cores. Based on the SD5 example, we think it is likely that the other crescent-shaped seafloor depressions formed originally as pockmarks by gas/fluid venting, and have since become inactive. The pockmarks represent zones of weakened sediment that were eroded, expanded, and merged by bottom currents to form larger seafloor depressions. Modern currents are strong enough to transport shelf sediments, and these currents were probably much stronger at lower sea levels when the Korea Strait was a more restricted passage between the East China Sea and East Sea.

Active tectonics of offshore Cide–Sinop (southern Black Sea shelf): from seismic and multibeam bathymetry data

Approximately 600 km of multichannel seismic reflection data and multibeam bathymetric data with an area of 2750 km2 indicate that offshore Cide–Sinop is filled by the sediments of four main seismic units dating from the Upper Cretaceous–Paleocene to Plio-Pleistocene. The deposits from the Eocene to modern times are highly eroded from their upper surfaces and widely outcropped at the seafloor. These units constitute the large flat shelf plain with an average depth of 100 m. Moreover, there are no significant marine onlaps identified in the shelf area. These stratigraphical and morphological features indicate that, until very recently, the area was a terrestrial landscape. Structurally, the shelf-plain and shelf-slope are primarily deformed by E–W-trending active reverse faults and strike–slip faults with a compressional component. These faults are densely spaced toward the outer shelf as fault segments bordering E–W-oriented dunes on the seafloor. A strike–slip fault with a reverse component delimits the shelf edge at long distances. Toward offshore Inebolu, the fault character changes to a negative flower structure that bends to the WNW direction, which indicates that the strike–slip fault is right lateral. The strike–slip faulting is also effective along the shelf slope deforming the slumps. These tectonic structures demonstrate that the shelf area is widely deformed by E–W-trending active transpressional strike–slip faults and reverse faults. This result reveals that the southern Black Sea shelf is under a recent N–S compressional tectonic regime.

Distribution of Crustal Melt Bodies at the Hot Spot‐Influenced Section of the Galápagos Spreading Centre From Seismic Reflection Images

AbstractAccretion of the lower crust at mid‐ocean ridges is a debated topic, with modern seismic observations pointing to a complex magmatic system that includes an axial multisill system of middle‐ and lower‐crustal melt lenses and near‐ and off‐axis melt bodies. Here we revisit the hot spot‐influenced section of the western Galápagos Spreading Centre and reprocess multichannel seismic reflection data using a wide‐angle seismic tomography model. Our new images show that the magma reservoir in the lower crust at this ridge section is intruded with partially molten melt lenses. The images also show evidence for off‐axis melt lenses, magmatic‐hydrothermal interactions and Moho reflections in this region. We conclude that the similarities between the axial crustal structure of this hot spot‐influenced mid‐ocean ridge and the multisill magmatic structure imaged at the East Pacific Rise indicate that these features are common along the global mid‐ocean ridge system where seafloor spreading is dominated by magmatic accretion.