Bathymetric Depression Research Articles

3D depositional architecture of a wave-dominated delta in a far-field GIA region: The case of the São Francisco delta, Brazil

The São Francisco delta in eastern Brazil is considered a classical example of a wave-dominated delta constructed in a glacial isostatic adjustment (GIA) far-field region. Although this delta is widely mentioned in the international literature, few details about its sedimentary architecture have been published. In this paper, we use a new dataset consisting of 15 boreholes and 26 radiocarbon datings to discuss the 3D architecture of the delta and the role played by GIA far-field effects in delta construction. The delta plain consists of a 10–12 m thick littoral sand sheet that progrades over a muddy subaqueous clinoform, forming a compound clinoform. Delta construction began between 8.4 and 7.2 ka BP and included a stage of normal regression that lasted until 5.7 ka BP, followed by a stage of forced regression. The transition between these two stages is marked by the highest position of the sand–mud transition in the boreholes. This forced regression is due to a 3–4-m drop in relative sea level since the mid-Holocene, which is characteristic of GIA far-field regions. Additionally, as a consequence of this drop in relative sea level, there is a progressive increase in the altitude of the beach-ridge deposits continentward, which combined with the development of extensive dune fields in the later stages of delta construction, contributes to the increased resilience of the delta to ongoing sea-level rise.The 3D architecture of the delta was strongly influenced by antecedent topography. The delta was built on a bathymetric depression on the shelf that almost entirely traps the sediment load of the São Francisco River. The uneven relief inside this bathymetric depression ultimately controlled lateral variations in the total thickness of the delta edifice.

A possible Jurassic age for the New Caledonia Trough and implications for Zealandia’s history

ABSTRACT The New Caledonia Trough is a bathymetric depression extending 2,500 km from New Zealand to the Coral Sea. Sedimentary successions up to 10 km thick are contained in three main sedimentary basins, the New Caledonia, Fairway and Deepwater Taranaki basins. Deepwater Taranaki Basin occupies the head of the New Caledonia Trough adjacent to the well-known Taranaki Basin lying almost entirely within New Zealand’s marine territory. The inboard part of Deepwater Taranaki Basin is covered by a dense grid of high-quality 2D and some 3D seismic data and one exploration well, Romney-1, which did not drill the syn-rift succession. The oldest sedimentary rocks drilled by Romney-1 are palynologically dated as Teratan (latest Turonian and Coniacian – 91–87 Ma). While the New Caledonia Trough is generally thought to have been initiated by Late Cretaceous (Campbell and Mortimer 2014; Strogen et al. 2017), this paper examines the evidence for the age of the oldest sedimentary rocks in the Deepwater Taranaki Basin and hence its age of inception and by inference, inception of the New Caledonia Trough to suggest a possible Jurassic age.

Intensified intrusion and retention of saltwater in the Sumjin River Estuary, South Korea

The Sumjin River Estuary (SRE) is located in South Korea and is a natural riverine system without an estuarine barrage. Civil complaints have been expressed by residents surrounding the SRE, particularly with regards to the extended upstream saltwater intrusion and the diminished production of the Corbicula japonica clam. To establish a scientific background of the complaints, more than 75 surveys across the SRE were performed, including gathering of long-term CTD monitoring data in the middle reaches of the SRE during 2015–2017. Although salinity at the mooring station normally fluctuates between zero and 20 psμ with tidal variation, the salty (>15 psμ) bottom water is consistently retained in the bathymetric depression of the middle reaches of the SRE, the main habitat for the benthic C. japonica clam, particularly during the neap tide and periods of moderate freshwater discharge. The maximum saltwater intrusion distance upstream of the estuary increased from 22.6 km to 24 km during the decade between 2005 and 2015. In addition, saltwater intrusions greater than 20 km have become more frequent over the decade. The increased utilization of freshwater in the upstream river reaches has been considered a primary constraint of the intensified saltwater intrusion in the SRE. This is enhanced by freshwater abstractions from an intake located near the SRE, particularly during low precipitation periods. The saltwater retention on the river bottom and the intensified saltwater intrusion upstream imply that estuarine ecosystem management policies are needed to incorporate salinity monitoring near the river bottom, where benthic organisms occur. The abstractions at the upstream freshwater intake are also recommended to be carefully controlled, particularly during the dry season.

Beryllium isotope variations recorded in the Adélie Basin, East Antarctica reflect Holocene changes in ice dynamics, productivity, and scavenging efficiency

The Adélie Basin is a relatively small (∼1600 km2), semi-enclosed continental shelf bathymetric depression located adjacent to the Wilkes Subglacial Basin, a basin underlying a sector of the East Antarctic Ice Sheet that contains ∼3–4 m sea level equivalent of ice. Located within the Adélie Basin is a ∼184 m thick laminated sediment deposit, the Adélie Drift, ideal for examining regional changes in ice sheet and ocean dynamics. Here, we examine the ratio of reactive beryllium-10 to reactive beryllium-9 ((10Be/9Be)reac) in a marine sediment core obtained from the Adélie Drift to assess these changes during the Holocene epoch (11.7 ka BP to present). The (10Be/9Be)reac record provides insight into changes in freshwater input, primary productivity, and scavenging efficiency, while removing the influence of particle size on 10Be concentration. During the early Holocene, (10Be/9Be)reac ratios indicate increased meltwater discharge from ca. 11.7 to 10 ka BP, as grounded ice retreated from the Adélie Basin and adjacent bathymetric highs. After ∼10 ka BP, beryllium isotopes are influenced by scavenging efficiency and dilution controlled by ocean currents and accumulation rate, operating alongside meltwater input, suggesting there are additional factors to consider when using (10Be/9Be)reac as a proxy for ice shelf cover and glacial dynamics.

Hualien Ridge: A tectonic ridge transitioning from plate collision to subduction

The Longitudinal Valley in eastern Taiwan is generally considered as a collisional suture between the Philippine Sea and the Eurasian plates. Offshore to the northeast of the valley, the Philippine Sea Plate is subducting beneath the Ryukyu Arc. The corner between eastern Taiwan and the Ryukyu Arc system is therefore the transition from plate collision to plate subduction. In consequence, the area has complicated tectonics and frequent earthquakes. In this study, we used marine geophysical data to study the submarine Hualien Ridge that is situated in this plate collision/subduction transition zone. Our results show that the Hualien Ridge is tectonically divided into the active southern part and the inactive northern part. In the southern Hualien Ridge, we find several ~N30°E trending active faults and some could be linked to the active faults in the onshore Milun Tableland. The structures in the southern Hualien Ridge and the Milun Tableland display a pop-up structure that is subject to the oblique compression from the northwestward motion of the Philippine Sea Plate. The ~N30°E trending faults are the results of the transpressional system. However, the Milun Fault, the western boundary of the fault system, probably terminates near 24°03′N, where a pronounced bathymetric depression trending N300° cuts across the whole Hualien Ridge. In fact, all the active faults in the southern Hualien Ridge only appear to the south of the bathymetric depression. In contrast, in the northern Hualien Ridge we only find blind normal faults covered by ~100 m thick sediments. The distinct variation of tectonic activity in the Hualien Ridge underlines the transition from the active collision to inactive collision or partial subduction of the Philippine Sea Plate relative to the Eurasian Plate.

Heat Flux From a Vapor‐Dominated Hydrothermal Field Beneath Yellowstone Lake

AbstractWe report results from 149 heat flux measurements made over an ∼2‐year interval at sites in and around a vapor‐dominated geothermal field located at water depths of ∼100–120 m in Yellowstone Lake, Wyoming. Measurements of both in situ temperature and thermal conductivity as a function of depth were made with a 1 m probe via a remotely operated vehicle, and are combined to compute the vertical conductive heat flux. Inside the ∼55.5 × 103 m2 bathymetric depression demarcating the vapor‐dominated field, the median conductive flux is 13 W m−2, with a conductive output of 0.72 MW. Outside the thermal field, the median conductive flux is 3.5 W m−2. We observed 49 active vents inside the thermal field, with an estimated mass discharge rate of 56 kg s−1, a median exit‐fluid temperature of 132°C, and a total heat output of 29 MW. We find evidence for relatively weak secondary convection with a total output of 0.09 MW in thermal area lake floor sediments. Our data indicate that vapor beneath the thermal field is trapped by a low‐permeability cap at a temperature of ∼189°C and a depth of ∼15 m below the lake floor. The thermal output of the Deep Hole is among the highest of any vapor‐dominated field in Yellowstone, due in part to the high boiling temperatures associated with the elevated lake floor pressures.

Seasonal shifts of microbial methane oxidation in Arctic shelf waters above gas seeps

AbstractThe Arctic Ocean subseabed holds vast reservoirs of the potent greenhouse gas methane (CH4), often seeping into the ocean water column. In a continuously warming ocean as a result of climate change an increase of CH4 seepage from the seabed is hypothesized. Today, CH4 is largely retained in the water column due to the activity of methane‐oxidizing bacteria (MOB) that thrive there. Predicted future oceanographic changes, bottom water warming and increasing CH4 release may alter efficacy of this microbially mediated CH4 sink. Here we investigate the composition and principle controls on abundance and activity of the MOB communities at the shallow continental shelf west of Svalbard, which is subject to strong seasonal changes in oceanographic conditions. Covering a large area (364 km2), we measured vertical distribution of microbial methane oxidation (MOx) rates, MOB community composition, dissolved CH4 concentrations, temperature and salinity four times throughout spring and summer during three consecutive years. Sequencing analyses of the pmoA gene revealed a small, relatively uniform community mainly composed of type‐Ia methanotrophs (deep‐sea 3 clade). We found highest MOx rates (7 nM d−1) in summer in bathymetric depressions filled with stagnant Atlantic Water containing moderate concentrations of dissolved CH4 (< 100 nM). MOx rates in these depressions during spring were much lower (< 0.5 nM d−1) due to lower temperatures and mixing of Transformed Atlantic Water flushing MOB with the Atlantic Water out of the depressions. Our results show that MOB and MOx in CH4‐rich bottom waters are highly affected by geomorphology and seasonal conditions.

Data-driven insights into correlation among geophysical setting, topography and seafloor sediments in the Ross Sea, Antarctic

Detailed mapping based on the high-resolution grids, such as GEBCO, ETOPO1, GlobSed, EGM-2008 is crucial for various domains of Earth sciences: geophysics, glaciology, Quaternary, sedimentology, geology, environmental science, geomorphology, etc. The study presented a GMT-based scripting techniques of the cartographic data processing aimed at the comparative analysis of the bathymetry, sediment thickness, geologic objects and geophysical settings in the study area based on various datasets. The study area is located in the Ross Sea, Antarctic. The highest values of the sediment thickness over 7,500 m are dominating in the southwest segment of the Ross Sea closer to the Victoria Land, followed by the region over the Ross Ice Shelf with values between 5,500 to 7,000 m (170°-175°W). The increased sediment thickness (2,500 to 3,000 m) was also mapped seen in the region NE off the Sulzberger Bay (70-75°S to 140-155°W), caused by the closeness of the Marie Bird Land ice coasts. A remarkable correlation between the gravity and the topography of the sea-land border in the Marie Bird Land area is well reflected in the coastal line and a set of the higher values in the free-air gravity. On the contrary, negative values (–60 to -80 mGal) are notable along the submarine toughs stretching parallel in the western part of the basin: e.g. the trough stretching in NW-SE direction in the 170°W-175°E, 65°S-68°S, between the 167°W-175°W, 70°S-72°S. Such correlations are clearly visible on the map, indicating geological lineaments and bathymetric depressions correlating with gravity grids. The paper contributes to the regional studies of the Ross Sea, the Antarctic and Polar region, and development of the cartographic technical methodologies by presenting an application of the GMT for thematic mapping.

Waves in Collections of Circular Shoals and Bathymetric Depressions

Abstract New approximate analytic solutions are developed to study wave propagation through collections of coastal features. Solutions are developed for circular shoals formed by a submerged cylind...

Magmatism versus serpentinization—crustal structure along the 13°N segment at the Mid-Atlantic Ridge

SUMMARY A region of oceanic core complexes (OCCs) exists at 13°N on the Mid-Atlantic Ridge that is regarded as a type site. This site includes two OCCs at 13°20′N and 13°30′N, thought to be in the active and dying stages of evolution, and two together called the Ashadze Complex (centred at 13°05′N) that are considered to be relict. Here we describe the results of S-wave seismic modelling along an ∼200-km-long 2-D transect traversing, south-to-north, through both the Mercurius and Marathon fracture zones, the southern outside corner of the 13°N segment, the OCCs, the ridge axis deviation in trend centred at 13°35′N, and the youngest oceanic crust of the eastern ridge flank to the north. Our inversion model, and the corresponding Vp/Vs ratio, show that the majority of the crust beneath the 13°30′N OCC comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level, while basaltic lithologies underlie the 13°20′N OCC. The transition between these contrasting crustal structures occurs over a distance of <5 km, and extends to at least ∼2 km depth below seafloor. The northern and southern OCCs of the Ashadze Complex have contrasting structures at shallow depth, with the northern OCC having a faster S-wave velocity in the upper crust. A Vp/Vs ratio of >1.9 (and equivalent Poisson's ratio of >0.3) indicates exhumed and/or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern OCC. Between the northern and southern flanks of the Marathon fracture zone and northern flank of Mercurius fracture zone, the lower crust has a relatively low Vp/Vs ratio suggesting that the deformation associated with Marathon fracture zone, which facilitates fluid ingress, extends laterally within the lower crust. Marathon fracture zone itself is underlain by a broad zone of low S-wave velocity (∼2.0 km s−1) up to ∼20 km wide from the seabed to at least the mid-crust, that is mirrored in a high Vp/Vs ratio and lower density, particularly deeper than ∼1 km below seabed within its bathymetric footprint. Volcanic domains are highlighted by a low Vp/Vs ratio of <1.6 (and equivalent Poisson's ratio of <0.15). Our combined seismic and density models favour the localized model of OCC evolution. They also show a considerable ridge-parallel variability in the amount and distribution of magmatic versus metamorphosed crust. Our results suggest that the current focus of magmatism lies to the north of the 13°20′N OCC, where the magmatic accretion-type seabed morphology observed is mirrored in the pattern of microseismicity, suggesting that its inward-facing median-valley-wall fault may link to the 13°20′N OCC detachment surface. Magmatism and active faulting behind (to the west) the footwall breakaway of the 13°30′N OCC, and the microseismicity concentrated in a band along its southern flank, suggest a readjustment of ridge geometry along axis is underway. As part of this, a transform offset is forming that will ultimately accommodate the 13°30′N OCC in its inside corner on the eastern flank of the ridge axis to the north.

Strike-slip related folding within the Malvinas/Falkland Trough (south-western Atlantic ocean)

The Malvinas/Falkland Trough (M/FT) is an E-W trending bathymetric depression along which runs the present-day South American-Scotia plate boundary. To the north of the M/FT lies the southern sector of the South Malvinas/Falkland Basin, and to the south lies the Burdwood Bank, an elongated morphological high constituting part of the South Scotia Ridge. Analysis of bathymetric and slope maps, integrated with seismic reflection profiles, have allowed describing in detail the M/FT in the sector comprised between 60° and 57° W. Data show the presence of an array of folds forming a thin-skinned fold-and-thrust Belt (FTB), previously interpreted as an active compressional field. The thin-skinned FTB is developed in a triangular-shaped area, which extends ca. 100 km in the E-W direction and 16 km in the N-S direction. Four parallel, asymmetric folds that develop to progressively greater water depths (between 1600 and 2600 m depth) towards the north, with orientations ESE-WNW and lengths that vary between 22 and 98 km, have been recognized. These folds are constituted by Late Cenozoic sediments that are undeformed in the Malvinas/Falkland Trough. The development of folds, which began in the late Miocene (ca. 7 Ma), was associated with the left-lateral strike-slip tectonic regime of the Magallanes-Fagnano-Malvinas lineament, the western segment of the South American-Scotia plate boundary.

Constraints on crustal structure of adjacent OCCs and segment boundaries at 13°N on the Mid-Atlantic Ridge

The 13° N segment of the Mid-Atlantic Ridge is an example of a morphologically well-studied slow spreading ridge segment populated with oceanic core complexes (OCCs). In this paper we present the results of a ∼200 km-long 2D seismic and gravity transect through this segment, the bounding fracture zones to the south and the ridge discontinuity to the north. We use this transect to consider the two end-member models of OCC evolution in which one, referred to as the Segment-scale model, implies they are interconnected with their detachments being part of a single segment-long feature, and the other, the Localised model, that each OCC is structurally isolated. We show, using the 7.5 km s−1 velocity contour as the base of crust marker, that the crust is consistently relatively thin ridge-parallel, at ∼5 km-thick on average, and that, beneath the OCCs, the Moho marks the top of a velocity gradient transition into the mantle, rather than a distinct velocity discontinuity. Although each OCC is not traversed in an identical structural location, they show a different crustal velocity-density structure with depth, with along axis variations in this structure mirrored by the bathymetric deeps between them. Older OCCs have a contrasting velocity-depth signature to the currently active 13°20′N OCC. The 13°20′N OCC is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours, while the 13°30′N OCC has an apparently thinner crust. Our combined P-wave seismic travel-time tomography and gravity forward modelling suggests that the OCCs of the 13° N segment are not interconnected at depth. To the north of the 13°30′N OCC, our modelling also suggests that the crust is being magmatically refreshed, or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge. The profile also crosses the Marathon and Mercurius fracture zones that mark the southern limit of the 13° N segment and the southern ridge-transform intersection outside corner. Along profile, Marathon fracture zone offsets younger (∼1 My vs ∼8 My) oceanic crust than Mercurius fracture zone (∼8 My vs ∼11 My). When considered in combination, both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of Marathon fracture zone, coupled with a region of low density that, most likely, reflects serpentinisation of the uppermost mantle. In addition, the crust captured between fracture zones appears relatively rotated about an E-W axis and uplifted to the north, with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre. Both the outside corner and the crust bounded by fracture zones have velocity-depth characteristics similar to that of the 13° N segment OCCs rather than normally-accreted oceanic crust, particularly in the upper-to-middle crust. Overall, our results support the Localised model of OCC evolution and suggest that fracture zones do not become locked immediately on transform-to-fracture transition as current models dictate.

Occurrence of ice-rafted erratics and the petrology of the KR1 seamount trail from the Australian–Antarctic Ridge

ABSTRACTA multi-disciplinary study of the KR1 segment of the Australian–Antarctic Ridge has been conducted since 2011. We present geochemical and age dating results for samples dredged from three sites on the KR1 seamount trail. The majority of the samples are alkaline ocean island basalts with subdominant enriched tholeiites. The samples from the DG05 bathymetric depression include ice-rafted erratics from Antarctica, which consist of gabbro, diabase, various granitoids, volcanic rocks such as trachyte and rhyolite and deformed or undeformed sedimentary rocks. The main provenance of glacial erratics is considered to be the Ross Sea region. However, Carboniferous to Cretaceous ages of erratics indicate that some of these may originate from the western regions of West Antarctica. Based on the size and topography of the volcanic features and geochemical characteristics of the alkaline ocean island basalts (La/SmN = 2.62–3.88; Tb/YbN = 1.54–2.67) and the enriched tholeiites, the KR1 seamount trail is interpreted to be a submarine hotspot chain that is the product of alkaline volcanic eruption and seafloor spreading.

Coupling biophysical processes that sustain a deep subpopulation of Loxechinus albus and its associated epibenthic community over a bathymetric feature

In order to contribute with more integrative strategies for better identify areas of high conservation priority we study the coupling of physical and biological processes that sustain a deep subpopulation of the most caught sea urchin (Loxechinus albus) worldwide and its associated epibenthic community over a bathymetric depression in southern Chile, which is surrounded by intensive anthropic activities. We used a remotely operated vehicle (ROV) to characterize the epibenthic community (organisms > 15 mm long) and substrate, up to a depth of 112 m in a submarine hole. Additionally, a combination of a towed and a moored Acoustic Doppler current Profiler (ADCP) was used to characterize current patterns. We found evidence that physical mechanisms transport and retain macro and micro material in the bottom of the hollow, and they contribute to homogenizing the habitat along the entire bathymetric gradient. These coupled processes allow i) that several species expand their bathymetric distribution range, ii) the existence of nursery areas for fishes, iii) that characteristic grazers in shallow habitats colonize greater depths, where filter feeders or DOM consumers generally represent the typical epifauna. We hypothesize that coupling of physical and biological processes on this bathymetric depression are key for bring on Loxechinus albus along the entire bathymetric gradient, who in turn is key to the community structure of the hollow. Given that this particular habitat is surrounded by intensive anthropic activities (eg. salmon aquaculture, mussel aquaculture, fishing, port) and a city, we suggest: Firstly, that monitoring should be undertaken in order to observe biodiversity changes in the bathymetric gradient, since it could be an indicator of conservation status. Secondly, that a proposal should be formulated to consider this zone as a Marine Protected Area.

Seismic evidence for arc segmentation, active magmatic intrusions and syn-rift fault system in the northern Ryukyu volcanic arc

Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic (MCS) reflection data. The study area forms an active volcanic front in parallel to the non-volcanic island chain in the eastern margin of the Eurasian plate and has been undergoing regional extension on its back-arc side. We carried out a MCS reflection experiment along two across-arc lines, and one of the profiles was laid out across the Tokara Channel, a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The reflection image reveals that beneath this topographic valley there exists a ~ 3-km-deep sedimentary basin atop the arc crust, suggesting that the arc segment boundary was formed by rapid and focused subsidence of the arc crust driven by the arc-parallel extension. Around the volcanic front, magmatic conduits represented by tubular transparent bodies in the reflection images are well developed within the shallow sediments and some of them are accompanied by small fragments of dipping seismic reflectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may suggest that the arc volcanism has multiple active outlets on the seafloor which bifurcate at crustal depths and/or that the location of the volcanic front has been migrating trenchward over time. Further distant from the volcanic front toward the back-arc (> 30 km away), these volcanic features vanish, and alternatively wide rift basins become predominant where rapid transitions from normal-fault-dominant regions to strike-slip-fault-dominant regions occur. This spatial variation in faulting patterns indicates complex stress regimes associated with arc/back-arc rifting in the northern Okinawa Trough.

GEOMORPHOLOGY, SEDIMENTOLOGY AND GEOCHEMISTRY IN THE MARINE AREA BETWEEN SIFNOS AND KIMOLOS ISLANDS, GREECE

An extensive oceanographic survey was conducted in the marine area between Kimolos and Sifnos Islands, a rather poorly-studied sector of the Aegean Sea, in order to gain better understanding of submarine geomorphological features and associated sediment provenance. Multi-beam bathymetry and surface sediment sampling with a box corer were carried out on board R/V Aegaeo, followed by grain-size analysis, XRD and XRF measurements. A large circular depression was identified north of Kimolos reaching a water depth of 743 m, filled with fine grained sediments. Surface sediment distribution is characterized by gradual decrease in grain-size from silty sand to silt in a S-N direction. The mineralogical composition comprises calcite, Mgcalcite, aragonite, dolomite, quartz, K feldspars, plagioclase, amphiboles and clay minerals. Major sediment provinces identified were: (i) the shallow sector proximal to Kimolos, characterized by higher contents in Si, Al, K, Rb and Ba; (ii) the deep area west of Sifnos, including the large depression, characterized by higher contents in Fe, Ti, Na, Mg, S, Cr, Cu, Ni, V, Zn; and (iii) the south passage between Kimolos and Sifnos, which exhibits higher Ca, S, and Sr contents. Manganese enrichment was observed in and around the bathymetric depression, where Mn oxides act efficiently as scavengers of a suite of metals.

Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate

The Ryukyu (Nansei-Shoto) island arc-trench system, southwest of Japan, is formed by the subduction of the Philippine Sea (PHS) plate. Among the subduction zones surrounding the Japan Islands, the Ryukyu arc-trench system is unique in that its backarc basin, the Okinawa Trough, is the area with current extensively active rifting. The length of the trench is around 1400 km, and the geological and geophysical characteristics vary significantly along the trench axis. We conducted multichannel seismic (MCS) reflection and wide-angle seismic surveys to elucidate the along-arc variation in seismic structures from the island arc to the trench regions, shooting seven seismic lines across the arc-trench system and two along-arc lines in the island arc and the forearc areas. The obtained P-wave velocity models of the Ryukyu arc crust were found to be heterogeneous (depending on the seismic lines), but they basically consist of upper, middle, and lower crusts, indicating a typical island arc structure. Beneath the bathymetric depressions cutting the island arc—for example, the Kerama Gap and the Miyako Saddle—the MCS record shows many across-arc normal faults, which indicates the presence of an extensional regime along the island arc. In the areas from the forearc to the trench, the subduction of the characteristic seafloor features on the PHS plate affects seismic structures; the subducted bathymetric high of the Amami Plateau is detected in the northern trench: the Luzon–Okinawa fracture zone beneath the middle and southern trenches. There are low-velocity (<~4.5 km/s) wedges along the forearc areas, except for off Miyako-jima Island. The characteristic high gravity anomaly at the forearc off Miyako-jima Island is caused not by a bathymetric high of a large-scale accretionary wedge but by shallower materials with a high P-wave velocity of ~4.5 km/s.Graphical Position of the seismic survey lines in the Ryukyu arc-trench system (right) and P-wave velocity models for the lines perpendicular to the trench axis (left). The contour interval is 2.5 km/s. The vertical blue line shows the position of the trench axis.

Chapter 13 Anatomy of the Andaman–Nicobar subduction system from seismic reflection data

Abstract The Andaman–Nicobar subduction system is the northwestern segment of the Sunda subduction system, where the Indian Plate subducts beneath the Sunda Plate in a nearly arc-parallel direction. The entire segment ruptured during the 2004 great Andaman–Sumatra earthquake ( M w =9.3). Using recently acquired high-resolution seismic reflection data, we characterize the shallow structure of the whole Andaman–Nicobar subduction system from west to east, starting from the nature of the subducting plate in the Bay of Bengal to back-arc spreading in the Andaman Sea. We find that the Ninety-East Ridge is overlain by thick continental margin sediments beneath the recent Bengal Fan sediments. The boundary between these two sedimentary units defines the plate interface. We observe evidence of re-activation of fracture zones on the subducting plate beneath the forearc, influencing the morphology of the upper plate. The forearc region, which includes the accretionary wedge, the forearc high and the forearc basin, is exceptionally wide (250 km). We observe an unusually large bathymetric depression within the forearc high. The forearc high is bounded in the east by a normal fault, whereas the forearc basin contains an active backthrust. The forearc basin is floored by the continental crust of Malayan Peninsula origin. The active sliver strike-slip fault lies in a deep basin, created during the rifting of the forearc continental crust and the Malayan Peninsula. The sliver fault connects with the Great Sumatra Fault in the south and with the Sagaing Fault in the north, via the Andaman Sea spreading centre and a large transform fault in the Andaman Sea.

Unreciprocated sedimentation along a mud-dominated continental margin, Gulf of Mexico, U.S.A.: Implications for sequence stratigraphy in muddy settings devoid of depositional sequences

According to widely accepted sequence stratigraphic and fill-and-spill models, sedimentary cyclicity along continental margins is modulated by relative sea-level change, whereas smaller-scale intraslope accommodation is controlled by the filling of pre-existing bathymetric depressions. Although these concepts are presumed to apply to shelf-to-slope settings regardless of grain size, we have tested both hypotheses in the mud-prone lower Pliocene to Holocene of offshore Louisiana, Gulf of Mexico, and reach different conclusions. We determine that over the last ∼3.7 Myr, differential accumulation and accompanying salt tectonism dislocated the fine-grained shelf and slope, prevented the development of sedimentary reciprocity at 10–100 kyr time scales, and inhibited fill-and-spill accumulation. We show that only 3% of “lowstand” mass transport deposits can be correlated to low stands in relative sea level, whereas approximately 30% of the deposits are related to transgressions and high stands; the remaining 67% are poorly constrained. Mass transport deposits also show no clear evidence of up-section increases in bypass. Based on our results, we conclude that the dominant control on stratigraphic architecture in offshore Louisiana was not relative sea-level change or patterns of accommodation, but rather differential deposition and concomitant salt-related subsidence, which controlled the distribution of facies, timing and location of mass transport deposits, and rates of sediment accumulation. Our conclusions highlight the importance of sediment supply and local tectonism, and caution against a priori use of conventional sequence stratigraphic and fill-and-spill models to decipher the stratigraphic evolution of actively-deforming mud-dominated continental margins. We therefore recommend treating stratigraphic models as testable hypotheses, rather than as methods of interpretation, particularly in fine-grained areas devoid of well-developed depositional sequences and in settings lacking intraslope ponded-to-perched accumulations.

Flow patterns and morphology of a prograding river delta

AbstractThe transition of flow between laterally confined channels and the unchannelized delta front controls the morphodynamic evolution of river deltas but has rarely been measured at the field scale. We quantify flow patterns and bathymetry that define the evolution of the subaqueous delta front on the Wax Lake Delta, a rapidly prograding delta in coastal Louisiana. A significant portion of flow (∼59%) departs the channel network over lateral channel margins as opposed to the downstream channel tips. Bathymetric surveys and remotely sensed estimates of flow direction allow spatial changes in flow velocity to be quantified and patterns of erosion and deposition to be estimated. Shallowing along channel margins produces spatial acceleration and erosion. Lateral spreading, deceleration, and deposition occur within three to eight channel widths outside of the channel margins. In interdistributary bays, the shape of each flow path is constrained by “nourishment boundaries” that separate the outflows from neighboring channels. Deposit elevation decreases with a basinward slope of 2.4 × 10−4 with distance from a channel margin along any flow path, regardless of the channel or location that flow departed the network. Bathymetric depressions called “interdistributary troughs” form along nourishment boundaries where flow paths are the longest and deposit elevation is correspondingly low. We conclude that the deposit morphology exerts a strong control on bathymetric evolution and that interaction between neighboring channels and even neighboring deltas can influence delta front morphology.