Multichannel Seismic Data Research Articles

The nature of crustal reflectivity at the southwest Iberian margin

Reprocessing of multi-channel seismic reflection data acquired over the northern margin of the Gulf of Cádiz (SW Iberian margin) places new constraints on the upper crustal structure of the Guadalquivir-Portimão Bank. The data presented have been processed with optimized stacking and interval velocity models, a better approach to multiple attenuation, preserved amplitude information to derive the nature of seismic reflectivity, and accurate time-to-depth conversion after migration. The reprocessed data reveal a bright upper crustal reflector just underneath the Paleozoic basement that spatially coincides with the local positive free-air gravity high called the Gulf of Cádiz Gravity High. To investigate the nature of this reflector and to decipher whether it could be associated with pieces of mantle material emplaced at upper crustal levels, we calculated its reflection coefficient and compared it to a buried high-density ultramafic body (serpentinized peridotite) at the Gorringe Bank. Its reflection coefficient ratio with respect to the sea floor differs by only 4.6% with that calculated for the high-density ultramafic body of the Gorringe Bank, while it differs by 35.8% compared to a drilled Miocene limestone unconformity. This means that the Gulf of Cádiz reflector has a velocity and/or density contrast similar to the peridotite at the Gorringe Bank. However, considering the depth at which it is found (between 2.0 and 4.0km) and the available geological information, it seems unlikely that the estimated shortening from the Oligocene to present is sufficient to emplace pieces of mantle material at these shallow levels. Therefore, and despite the similarity in its reflection coefficient with the peridotites of the Gorringe Bank, our preferred interpretation is that the upper crustal Gulf of Cádiz reflector represents the seismic response of high-density intracrustal magmatic intrusions that may partially contribute to the Gulf of Cádiz Gravity High.

Characteristics and generation mechanism of gullies and mega-pockmarks in the Zhongjiannan Basin, western South China Sea

The paleoceanography and deep-water circulation process of the South China Sea (SCS) is still poorly understood. We have evaluated multibeam bathymetry and multichannel seismic reflection data acquired by Guangzhou Marine Geological Survey in recent years, and we investigate the characteristics, distribution, and generation mechanism of submarine seabed gullies and mega-pockmarks in the northern Zhongjiannan Basin of the SCS. Our data reveal that there are northwest–southeast-trending gullies and randomly or linearly arraying mega-pockmarks in the northern depression of the Zhongjiannan Basin. Gullies are 1 km or more in width and tens of kilometers in length. The long-axis diameter for the largest mega-pockmarks can reach up to 4293 m. The mega-pockmarks found in the west block of the northern depression are the manifestation of focused fluid flow migrating along shallow inclined faults induced by slumps that reached the seabed. Gullies are trending southeast and perpendicular to the contours of the continental slope, suggesting that gravity flows have played an important role in the evolution of gullies. Our data demonstrate that the gullies have a close relationship with mega-pockmarks, and that they were formed through the interaction of intense focused fluid flow and gravity flows. The pockmarks formed first, subsequently growing in size and number under the effect of subsurface fluid flows. The southeastward moving gravity flow caused discrete pockmarks to coalesce and form immature gullies. After being fully converged, gullies finally become smooth-bottomed and matured. Because gullies and mega-pockmarks are the manifestation of the geologic process of fluid escape at the seabed, they should be taken into account when assessing the potential for gas hydrates and seafloor instability.

Magmatism in the evolution of the South China Sea: Geophysical characterization

Timing, distribution, and intensity of magmatism are critical to understanding deep lithospheric processes in the opening of the South China Sea (SCS). Based primarily on multichannel seismic reflection data, and aided with gravity and magnetic anomalies, we identify various igneous features, such as sills, stocks, volcanic edifices, and volcanoes, which are often characterized by high positive seismic amplitudes, and/or by chaotic or blank reflections beneath them because of their seismic shielding effect. We estimate the timing of igneous activities and analyze their areal distribution in the SCS region based on seismic stratigraphy. We confirm that most igneous emplacements in the SCS margins occurred after the cessation of seafloor spreading, and are sparse and volumetrically small during the rifting and seafloor spreading phases, supporting magma-poor margins prior to the breakup of the continental lithosphere. By contrast, post-spreading magmatic activities are widespread in the continental slope areas and in the central SCS basin, and are most likely triggered by extension in relation to cooling and subsidence of the oceanic and attenuated continental lithosphere, which could accumulate a total thermal contractional displacement up to 24km.

Sediment waves in the Caroline Basin suggest evidence for Miocene shifts in bottom water flow in the western equatorial Pacific

We describe abyssal sediment waves in seismic profiles from the Caroline Basin, western Pacific Ocean, which indicate bottom currents at this location have shaped the seabed beginning in the early to mid-Miocene. Our evidence is based on 2D high-resolution multi-channel seismic data we collected in 2013 during a site survey for IODP Expedition 363. Drilling within this grid (at IODP Site U1490) was completed in late 2016 and we expect results will provide the age and composition needed to evaluate the paleoceanographic implications of the geomorphologic history we describe. Our seismic grid reveals roughly 150ms (two-way travel time; roughly 135m) of sediment waves with 10–40ms wave heights (roughly 9–36m) and 1–2km wavelengths. These waves most likely accumulated in carbonate-rich sediments, based on DSDP Site 62 (450km to the south but lacking evidence of bottom currents in profiles or cores). The wave-like bedforms in our survey grid are independent of underlying structures and may have developed in response to an early Neogene intensification of bottom water flow documented at ODP Site 1124 in the SW Pacific (Carter et al., 2004). Steady growth and slow NW migration of the waves in the survey grid continued for much of the Miocene. Above ~250ms (roughly 212m) below the seafloor lateral migration halted and wave growth slowed. Today waves are present on the seafloor due to vertical aggradation along the flank of a gentle ridge at the northern edge of our survey grid. We propose this evolution of wave morphology is linked to global climatic events that drove thermohaline circulation throughout the world's oceans.

Crustal structure of the North Iberian continental margin from seismic refraction/wide-angle reflection profiles

The structure and geodynamics of the southern margin of the Bay of Biscay have been investigated from a set of 11 multichannel seismic reflection profiles, recorded also at wide angle offsets in an onshore-offshore network of 24 OBS/OBH and 46 land sites. This contribution focuses on the analysis of the wide-angle reflection/refraction data along representative profiles. The results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the Iberian Peninsula. Offshore, the crust has a typical continental structure in the eastern tip of the bay, which disappears smoothly towards the NW to reach crustal thickness close to 10km at the edge of the studied area (~45°N, 6°W). The analysis of the velocity-depth profiles, altogether with additional information provided by the multichannel seismic data and magnetic surveys, led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic. Typical oceanic crust has not been imaged in the investigated area. Onshore, the new results are in good agreement with previous results and document the indentation of the Bay of Biscay crust into the Iberian crust, forcing its subduction to the North. The interpreted profiles show that the extent of the southward indentation is not uniform, with an Alpine root less developed in the central and western sector of the Basque-Cantabrian Basin. N-S to NE-SW transfer structures seem to control those variations in the indentation degree.

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Crustal structure and sedimentation history over the Alleppey platform, southwest continental margin of India: Constraints from multichannel seismic and gravity data

The Alleppey Platform is an important morphological feature located in the Kerala-Konkan basin off the southwest coast of India. In the present study, seismic reflection data available in the basin were used to understand the sedimentation history and also to carry out integrated gravity interpretation. Detailed seismic reflection data in the basin reveals that: (1) the Alleppey Platform is associated with a basement high in the west of its present-day geometry (as observed in the time-structure map of the Trap Top (K/T boundary)), (2) the platform subsequently started developing during the Eocene period and attained the present geometry by the Miocene and, (3) both the Alleppey platform and the Vishnu fracture zone have had significant impact on the sedimentation patterns (as shown by the time-structure and the isochron maps of the major sedimentary horizons in the region). The 3-D sediment gravity effect computed from the sedimentary layer geometry was used to construct the crustal Bouguer anomaly map of the region. The 3-D gravity inversion of crustal Bouguer anomaly exhibits a Moho depression below the western border of the platform and a minor rise towards the east which then deepens again below the Indian shield. The 2-D gravity modelling across the Alleppey platform reveals the geometry of crustal extension, in which there are patches of thin and thick crust. The Vishnu Fracture Zone appears as a crustal-scale feature at the western boundary of the Alleppey platform. Based on the gravity model and the seismic reflection data, we suggest that the basement high to the west of the present day Alleppey platform remained as a piece of continental block very close to the mainland with the intervening depression filling up with sediments during the rifting. In order to place the Alleppey platform in the overall perspective of tectonic evolution of the Kerala-Konkan basin, we propose its candidature as a continental fragment.

Characteristics of Meso-Cenozoic Igneous Complexes in the South Yellow Sea Basin, Lower Yangtze Craton of Eastern China and the Tectonic Setting

The South Yellow Sea Basin is partially surrounded by the East Asian continental Meso-Cenozoic widespread igneous rocks belt. Magnetic anomaly and multi-channel seismic data both reveal the prevalent occurrence of igneous rocks. We preliminarily defined the coupling relation between magnetic anomalies and igneous rock bodies. Some igneous complexes were also recognized by using multi-channel seismic and drilling data. We identified various intrusive and extrusive igneous rock bodies, such as stocks, sills, dikes, laccoliths and volcanic edifice relics through seismic facies analysis. We also forecasted the distribution characteristics of igneous complexes. More than fifty hypabyssal intrusions and volcanic relics were delineated based on the interpretation of magnetic anomaly and dense intersecting multi-channel seismic data. It is an important supplement to regional geology and basin evolution research. Spatial matching relations between igneous rock belts and fractures document that extensional N-E and N-NE-trending deep fractures may be effective pathways for magma intrusion. These fractures formed under the influence of regional extension during the Meso-Cenozoic after the Indosinian movement. Isotopic ages and crosscutting relations between igneous rock bodies and the surrounding bedded sedimentary strata both indicate that igneous activities might have initiated during the Late Jurassic, peaked in the Early Cretaceous, gradually weakened in the Late Cretaceous, and continued until the Miocene. Combined with previous studies, it is considered that the Meso-Cenozoic igneous activities, especially the intensive igneous activity of the Early Cretaceous, are closely associated with the subduction of the Paleo-Pacific Plate.

Toward a robust workflow for deep crustal imaging by FWI of OBS data: The eastern Nankai Trough revisited

AbstractCrustal‐scale imaging by the full‐waveform inversion (FWI) of long‐offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping. Therefore, efficient crustal‐scale FWI requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion. Here we attempt to reprocess a challenging 2‐D ocean‐bottom seismometer (OBS) data set from the eastern Nankai Trough. The starting model is built by first‐arrival traveltime tomography (FAT), which is FWI assisted for tracking cycle skipping. We iteratively refine the picked traveltimes and then reiterate the FAT until the traveltime residuals remain below the cycle‐skipping limit. Subsequently, we apply Laplace‐Fourier FWI, in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion. These two multiscale levels are complemented by a layer‐stripping approach implemented through offset continuation. The reliability of the FWI velocity model is assessed by means of source wavelet estimation, synthetic seismogram modeling, ray tracing modeling, dynamic warping, and checkerboard tests. Although the viscoacoustic approximation is used for wave modeling, the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy. The revised FWI scheme produces a high‐resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data. This study opens a new perspective on the design of OBS crustal‐scale experiments amenable to FWI; however, a further assessment of the optimal OBS spacing is required for reliable FWI.

Seismic stratigraphy of sedimentary cover in the Podvodnikov Basin and North Chukchi trough

The multichannel seismic reflection data (MCS data) obtained in the Arktika-2014 expedition revealed the essential fact that must be taken into account by the tectonic model of the Central Arctic region. The Brookian, Lower Cretaceous, and Upper Jurassic unconformities are continuously traced from the North Chukchi offshore trough into the Podvodnikov Basin, indicating that the depocenter in the latter accumulated both Cretaceous sedimentary sequences and Early–Middle Mesozoic ones.

Constraints on melt content of off‐axis magma lenses at the East Pacific Rise from analysis of 3‐D seismic amplitude variation with angle of incidence

AbstractWe use 3‐D multichannel seismic data to form partial angle P wave stacks and apply amplitude variation with angle (AVA) crossplotting to assess melt content and melt distribution within two large midcrustal off‐axis magma lenses (OAMLs) found along the East Pacific Rise from 9°37.5′N to 9°57′N. The signal envelope of the partial angle stacks suggests that both OAMLs are partially molten with higher average melt content and more uniform melt distribution in the southern OAML than in the northern OAML. For AVA crossplotting, the OAMLs are subdivided into seven ~1 km2 analysis windows. The AVA crossplotting results indicate that the OAMLs contain a smaller amount of melt than the axial magma lens (AML). For both OAMLs, a higher melt fraction is detected within analysis windows located close to the ridge axis than within the most distant windows. The highest average melt concentration is interpreted for the central sections of the OAMLs. The overall low OAML melt content could be indicative of melt lost due to recent off‐axis eruptions, drainage to the AML, or limited mantle melt supply. Based on the results of this and earlier bathymetric, morphological, geochemical, and geophysical investigations, we propose that the melt‐poor OAML state is largely the result of limited melt supply from the underlying mantle source reservoir with smaller contribution attributed to melt leakage to the AML. We hypothesize that the investigated OAMLs have a longer period of melt replenishment, lower eruption recurrence rates, and lower eruption volumes than the AML, though some could be single intrusion events.

Dyke emplacement and crustal structure within a continental large igneous province, northern Barents Sea

Abstract We perform an integrated analysis of magnetic anomalies, multichannel seismic and wide-angle seismic data across an Early Cretaceous continental large igneous province in the northern Barents Sea region. Our data show that the high-frequency and high-amplitude magnetic anomalies in this region are spatially correlated with dykes and sills observed onshore. The dykes are grouped into two conjugate swarms striking oblique to the northern Barents Sea passive margin in the regions of eastern Svalbard and Franz Josef Land, respectively. The multichannel seismic data east of Svalbard and south of Franz Josef Land indicate the presence of sills at different stratigraphic levels. The most abundant population of sills is observed in the Triassic successions of the East Barents Sea Basin. We observe near-vertical seismic column-like anomalies that cut across the entire sedimentary cover. We interpret these structures as magmatic feeder channels or dykes. In addition, the compressional seismic velocity model locally indicates near-vertical, positive finger-shaped velocity anomalies (10–15 km wide) that extend to mid-crustal depths (15–20 km) and possibly deeper. The crustal structure does not include magmatic underplating and shows no regional crustal thinning, suggesting a localized (dyking, channelized flow) rather than a pervasive mode of magma emplacement. We suggest that most of the crustal extension was taken up by brittle–plastic dilatation in shear bands. We interpret the geometry of dykes in the horizontal plane in terms of the palaeo-stress regime using a model of a thick elastoplastic plate containing a circular hole (at the plume location) and subject to combined pure shear and pressure loads. The geometry of dykes in the northern Barents Sea and Arctic Canada can be predicted by the pattern of dilatant plastic shear bands obtained in our numerical experiments assuming boundary conditions consistent with a combination of extension in the Amerasia Basin sub-parallel to the northern Barents Sea margin and a mild compression nearly orthogonal to the margin. The approach has implications for palaeo-stress analysis using the geometry of dyke swarms.

Marine‐controlled source electromagnetic study of methane seeps and gas hydrates at Opouawe Bank, Hikurangi Margin, New Zealand

AbstractMarine controlled source electromagnetic (CSEM) data have been collected to investigate methane seep sites and associated gas hydrate deposits at Opouawe Bank on the southern tip of the Hikurangi Margin, New Zealand. The bank is located in about 1000 m water depth within the gas hydrate stability field. The seep sites are characterized by active venting and typical methane seep fauna accompanied with patchy carbonate outcrops at the seafloor. Below the seeps, gas migration pathways reach from below the bottom‐simulating reflector (at around 380 m sediment depth) toward the seafloor, indicating free gas transport into the shallow hydrate stability field. The CSEM data have been acquired with a seafloor‐towed, electric multi‐dipole system measuring the inline component of the electric field. CSEM data from three profiles have been analyzed by using 1‐D and 2‐D inversion techniques. High‐resolution 2‐D and 3‐D multichannel seismic data have been collected in the same area. The electrical resistivity models show several zones of highly anomalous resistivities (>50 Ωm) which correlate with high amplitude reflections located on top of narrow vertical gas conduits, indicating the coexistence of free gas and gas hydrates within the hydrate stability zone. Away from the seeps the CSEM models show normal background resistivities between ~1 and 2 Ωm. Archie's law has been applied to estimate gas/gas hydrate saturations below the seeps. At intermediate depths between 50 and 200 m below seafloor, saturations are between 40 and 80% and gas hydrate may be the dominating pore filling constituent. At shallow depths from 10 m to the seafloor, free gas dominates as seismic data and gas plumes suggest.

Imaging the Yellow Sea Bottom Cold Water from multichannel seismic data

The Yellow Sea Bottom Cold Water (YSBCW) is a large cold water mass lying in the deep part of the Yellow Sea during the warm season. We acquired multichannel seismic (MCS) data using an air gun source to image the structure of the YSBCW. The MCS data recorded reflections from sea water. The recognition of these reflections was confirmed by finite-difference seismic modeling in the frequency-domain. The seismic section from MCS data enabled discrimination of water masses distinctly separated by reflecting horizons. The structure of the water masses is fairly consistent with temperature-depth variations obtained using expandable bathythermograph (XBT) casts. The YSBCW is imaged as the lowermost water mass, maximally 40 m thick, that extends as a lens-like form along the sea bottom under the warm mixed layer. The correlation of XBT measurements and the seismic section indicates a rapid decrease in temperature from around 11 to 8 °C in the uppermost part of the YSBCW. A transition zone between the mixed layer and the underlying YSBCW is also defined. This transition zone has fairly uniform thickness of 14–18 m and marks an interval of rapid temperature drop, indicating vigorous thermal mixing. Our study demonstrates that MCS profiling is a useful and reliable tool for imaging fine structures in the shallow Yellow Sea.

Dual-vergence structure from multiple migration of widely spaced OBSs

The detailed structure of the northern Cascadia basin and frontal ridge region was obtained using data from several widely spaced ocean bottom seismometers (OBSs). Mirror imaging was used in which the downgoing multiples (mirror signal) are migrated as they provide information about a much larger area than imaging with primary signal alone. Specifically, Kirchhoff time migration was applied to hydrophone and vertical geophone data. Our results indicate remarkable structures that were not observed on the northern Cascadia margin in previous single-channel or multi-channel seismic (MCS) data. Results show that, in these water depths (2.0–2.5km), an OBS can image up to 5km on either side of its position on the seafloor and hence an OBS spacing of 5km is sufficient to provide a two-fold migration stack. Results also show the top of the igneous oceanic crust at 5–6km beneath the seafloor using only a small airgun source (120in.3). Specifically, OBS migration results clearly show the continuity of reflectors which enabled the identification of frontal thrusts and a main thrust fault. These faults indicate, for the first time on this margin, the presence of a dual-vergence structure. These kinds of structures have so far been observed in <0.5% of modern convergent margins and could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Reanalysis of previous MCS data from this region augmented the OBS migration results and further suggests that the vergence switches from seaward to landward around central Vancouver Island. Furthermore, fault geometry analyses indicate that the total amount of shortening accommodated due to faulting and folding is about 3km, which suggest that thrusting would have started at least ∼ 65ky ago.

Structure of the northern Bay of Bengal offshore Bangladesh: Evidences from new multi-channel seismic data

New multi-channel seismic data were acquired in the northern part of the Bay of Bengal and at the northernmost termination of the 90°E Indian Ridge offshore Bangladesh. This survey was coupled with a seismic refraction experiment indicating this offshore basin is here floored by a thinned (15 km thick) continental crust, injected by Mesozoic volcanism. This attenuated continental crust is interpreted as formed during Gondwana super-continent fragmentation during a syn-rift period. The dominant tectonic pattern is marked by NE-SW trending tilted blocks filled by syn-rift sediments clearly identified on seismic profiles. The uppermost part of this continental crust (3–4 km thick) shows a complex assemblage of dipping reflectors and west-facing tilted blocks injected by volcanic build-ups. The lower crustal sequence (11–12 km thick) does not reveal significant reflectors. This syn-rift fabric is attributed to the Mesozoic up to the Early Cretaceous by correlation with published seismic data along the eastern coast of India. Opposite normal faults vergency on the Indian and Burma sides indicate an asymmetrical rifting (simple shear) creating a wide COT on the Burma side and a short COT on the opposite Indian side, a geometry typical of continental crust stretching.This crustal fabric is overlain disconformably by a thin reflector attributed to the Late Cretaceous-Paleocene pelagic sequence deposited during India/Bay of Bengal drift phase, before the Cenozoic-India Asia collision marked by thick clastic sedimentation associated with the Ganges delta southward progradation.Below this delta, this Mesozoic rift closes axially and is affected by the incipient Late Miocene shortening of the Shillong Plateau. The NE-SW fabric of this attenuated crust might be traced southward to 15°N, close to magnetic chron 34, where steady state spreading of the Central Indian Ocean occurred.

Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system

Hydrothermal circulation at mid-ocean ridge volcanic segments extracts heat from crustal magma bodies. However, the heat source driving hydrothermal circulation in ultramafic outcrops, where mantle rocks are exhumed in low-magma-supply environments, has remained enigmatic. Here we use a three-dimensional P -wave velocity model derived from active-source wide-angle refraction-reflection ocean bottom seismometer data and pre-stack depth-migrated images derived from multichannel seismic reflection data to investigate the internal structure of the Rainbow ultramafic massif, which is located in a non-transform discontinuity of the Mid-Atlantic Ridge. Seismic imaging reveals that the ultramafic rocks composing the Rainbow massif have been intruded by a large number of magmatic sills, distributed throughout the massif at depths of ∼2–10 km. These sills, which appear to be at varying stages of crystallization, can supply the heat needed to drive high-temperature hydrothermal circulation, and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting. Our results demonstrate that high-temperature hydrothermal systems can be driven by heat from deep-sourced magma even in exhumed ultramafic lithosphere with very low magma supply.

Seismic vis-a-vis sonic attenuation in gas hydrate bearing sediments of Krishna-Godavari basin, Eastern Margin of India

Seismic velocities in the sediments containing gas hydrates show a marked increase compared to the background, intuitively implying that there would be a reduction in the seismic attenuation (Q−1) of such sediments. However, the attenuation measurements carried out in the sonic frequency range from various gas hydrate provinces show that there is a notable increase in seismic attenuation within the gas hydrate layers and the results obtained are counter-intuitive. In this work we try to compare the attenuation derived by applying frequency shift method to the multichannel seismic (MCS) and sonic data sets at the same location in the Krishna-Godavari (KG) basin. The role of complex geology in attenuating the seismic signal is also studied by generating synthetic seismic data for different geological models and by computing the corresponding attenuation. It has been found that the Q−1 obtained from the field seismic data compares well with the Q−1 computed for a simple layered geological model. The results indicate that the Q−1 (0.0029) from field seismic data is ∼4.3 times lower than Q−1 (0.0123–0.0125) obtained from the sonic data, implying that the thickness of the gas hydrate layer in the KG basin is not sufficient enough to average the bulk properties. Our results also indicate that there could be substantial contribution of the pore scale interaction to the observed attenuation.

Seismic stratigraphy and glacial cycles in the inland passages of the Magallanes Region of Chile, southernmost South America

We present results of a marine geophysical survey that imaged submarine sedimentary and geomorphic features of the main inland channels of the Magallanes Region of Chile, in southernmost South America. This region holds a remarkable terrestrial and marine record of glacial variations in the closest landmass to Antarctica. We report evidence for grounded ice along the main channels associated with two ice lobes that flowed from Cordillera Darwin, part of the Austral Andes, into Central Estrecho de Magallanes and Seno Almirantazgo. Multibeam data show drumlins and glacial lineations in Whiteside Channel the northern continuation of Seno Almirantazgo, and iceberg plough marks in Central Estrecho de Magallanes. Multichannel seismic data show a complex array of seismic facies, interpreted as representing two main types of sedimentary units: glacimarine sediments, corresponding to glacial periods, and pelagic sediments, corresponding to interglacial periods. We find that there are two interglacial-glacial cycles represented in this marine sedimentary record. Chronologically: 1) the youngest glacial unit corresponds to the last regional glaciation (~31–18ka); 2) these passageways deglaciated with the retreat from regional glacial limit ‘D’ (~17–18ka), and 3) there is no marine sedimentary or morphological evidence that the Magallanes glaciers readvanced into the main passageways during the Antarctic Cold Reversal (~14–13ka). These findings suggest that final deglaciation of southernmost South America was driven by the atmospheric and ocean warming that started ~18ka as recorded in Antarctic ice core records and sea surface temperature proxies offshore the Magallanes Region.

The structural elements and tectonics of the Lake Van basin (Eastern Anatolia) from multi-channel seismic reflection profiles

This study analyzed multi-channel seismic reflection data from Lake Van, Eastern Anatolia, to provide key information on the structural elements, deformational patterns and overall tectonic structure of the Lake Van basin. The seismic data reveal three subbasins (the Tatvan, northern and Ahlat subbasins) separated by structural ridges (the northern and Ahlat ridges). The Tatvan basin is a tilted wedge-block in the west, it is a relatively undeformed and flat-lying deep basin, forming a typical example of strike-slip sedimentation. Seismic sections reveal that the deeper sedimentary sections of the Tatvan basin are locally folded, gently in the south and more intensely further north, suggesting a probable gravitational “wedge-block” instability, oblique to the northern margin. The northern subbasin, bounded by normal oblique faults, forms a basin-margin graben structure that is elongated in a northeast-southwest direction. The east-west trending Ahlat ridge forms a fault-wedged sedimentary ridge and appears to offset by reverse oblique faults forming as a push-up rhomb horst structure. The Ahlat subbasin is a fault-wedged trough fill that is elongated in the west-east direction and appears as a horst-foot graben formed by the normal oblique faults. The northeast-southwest directed northern ridge is a faulted crestal terrace of a sublacustrine basement block. Its step-like morphology, in response to the downfaulting of the Tatvan basin, as well as its backthrusted appearance, indicates the normal oblique nature of the bounding faults.The lacustrine shelf and slope show distinctive stratigraphic features; progradational deltas, submerged fluvial channels, distorted and collapsed beddings and soft sediment deformation structures, characterizing a highly unstable nature of shelf caused by strong oblique faulting and related earthquakes. The faulting caused uplift of the Çarpanak spur zone, together with the northeastern Erek delta, deformation of deltaic structures and subsequently exposing the shelf and slope areas. The exposed areas are evident in the angular unconformity surface of the Çarpanak basement block with the northeastern Erek delta and thinned sediments. The uplift resulted in the asymmetric depositional emplacement of the southeastern delta that is controlled by a series of ramp anticlines/low angle reverse faults. The Deveboynu subbasin and Varis spur zone form wide fault-controlled depressions with thick sediments that are elongated in the north-south direction. These subbasins appear as a small pull-apart boundary formed by normal oblique faults at the western end of the southeastern delta.