A buried submarine canyon in the northwestern South China Sea: architecture,development processes and implications for hydrocarbon exploration

High-resolution multichannel seismic data enables the discovery of a previous, undocumented submarine canyon(Huaguang Canyon) in the Qiongdongnan Basin, northwestern South China Sea. The Huaguang Canyon with a NW orientation is 140 km in length, and 2.5 km to 5 km in width in its upper reach and 4.6 km to 9.5 km in width in its lower reach. The head of the Huaguang Canyon is close to the Xisha carbonate platform and its tail is adjacent to the Central Canyon. This buried submarine canyon is formed by gravity flows from the Xisha carbonate platform when the sea level dropped in the early stage of the late Miocene(around 10.5 Ma). The internal architecture of the Huaguang Canyon is mainly characterized by high amplitude reflections, indicating that this ancient submarine canyon was filled with coarse-grained sediments. The sediment was principally scourced from the Xisha carbonate platform. In contrast to other buried large-scale submarine canyons(Central Canyon and Zhongjian Canyon) in the Qiongdongnan Basin, the Huaguang Canyon displays later formation time,smaller width and length, and single sediment supply. The coarse-grained deposits within the Huaguang Canyon provide a good environment for reserving oil and gas, and the muddy fillings in the Huaguang Canyon have been identified as regional caps. Therefore, the Huaguang Canyon is a potential area for future hydrocarbon exploration in the northwestern South China Sea. The result of this paper may contribute to a better understanding of the evolution of submarine canyons formed in carbonate environment.     

A buried submarine canyon in the northwest South China Sea:architecture,development processes and implications for hydrocarbon exploration

High-resolution multichannel seismic data enables the discovery of a previous, undocumented submarine canyon(Huaguang Canyon) in the Qiongdongnan Basin, northwest South China Sea. The Huaguang Canyon with a NW orientation is 140 km in length, and 2.5 km to 5 km in width in its upper reach and 4.6 km to 9.5 km in width in its lower reach. The head of the Huaguang Canyon is close to the Xisha carbonate platform and its tail is adjacent to the central canyon. This buried submarine canyon is formed by gravity flows from the Xisha carbonate platform when the sea level dropped in the early stage of the late Miocene(～10.5 Ma). The internal architecture of the Huaguang Canyon is mainly characterized by high amplitude reflections, indicating that this ancient submarine canyon was filled with coarse-grained sediments. The sediment was principally scourced from the Xisha carbonate platform. In contrast to other buried large-scale submarine canyons(central canyon and Zhongjian Canyon) in the Qiongdongnan Basin, the Huaguang Canyon displays later formation time, smaller width and length, and single sediment supply. The coarse-grained deposits within Huaguang Canyon provide a good environment for reserving oil and gas, and the muddy fillings in Huaguang Canyon have been identified as regional caps. Therefore, Huaguang Canyon is potential area for future hydrocarbon exploration in the northwest South China Sea. Our results may contribute to a better understanding of the evolution of submarine canyons formed in carbonate environment.     

Seismic geomorphology,architecture and genesis of Miocene shelf sand ridges in the Pearl River Mouth Basin,northern South China Sea

Multiple stages of large-scale shelf sand ridges, including the shoreface-attached and the offshore types, have developed in the Miocene successions on the mid-shelf region of the Pear River Mouth Basin, northern South China Sea. Utilizing a high-quality 3D seismic data set, accompanying 2D seismic profiles and well logs, the morphology, architecture and genesis of these shelf sand ridges have been systematically investigated in this study. The ridges are of very large scale, with the largest one having a maximum height of 64 m, a width of more than 20 km and a length of 37 km within the 3D survey area. Being mound-shaped, they also display obvious asymmetry character, with the ridge crest preferentially located on the SE side. Three main internal components, including the ridge front, central ridge and the ridge tail, have been recognized through careful anatomy analysis of the two most well-imaged ridges, each displaying distinct expressions on seismic amplitudes and geometries. In the plan view, most of the shelf sand ridges are generally NE–SW oriented and widening to the SW direction. Scouring features can also be clearly observed along the SW direction, including scour depressions and linear sandy remnants. On well logs, the shelf sand ridges are represented by an overall coarsening-upward pattern. Intervals with blocky sandstones are preferentially present on higher locations due to a differential winnowing process controlled by shelf topography.Plenty of evidence indicates that these ridges were primarily formed by the reworking of forced regressive or lowstand deltaic deposits under a persistent southwesterly flowing current during the subsequent transgression. This very current is a composite one, which is speculated to consist of winter oceanic current, SCSBK (South China Sea Branch of Kuroshio) intrusion onto the shelf and internal waves propagating from the Luzon Strait. Tidal currents might have contributed to the SE growth of the ridge. In response to the reglaciation of Antarctic ice-sheet and the closure of Pacific-Indian ocean seaway in the middle Miocene, the intensification of the North Pacific western boundary current was considered to have potential links to the initiation of the shelf sand ridges at ∼12 Ma. The development of shelf ridges was terminated and replaced by rapid deltaic progradation at ∼5.5 Ma.     

琼东南盆地北礁凹陷梅山组顶部丘形反射特征及成因分析

南海琼东南盆地北礁凹陷中中新统梅山组顶部丘形反射目前引起广泛关注,前人推测为生物礁、重力蠕动与底流叠加成因、等深积丘等。本文通过钻井资料、二维、三维地震资料精细刻画丘形反射（残丘）和丘间水道特征及其成因。残丘及水道在北礁凸起不发育,在边缘斜坡中部和高地较发育,且有向高地两边规模减小趋势,不具对称性,残丘和水道呈平行-亚平行近E-W向展布,局部有合并分叉现象,与北礁凸起走向呈一小角度;丘宽562~1 223 m,丘高29~87 m,丘长10 km左右,存在丘翼削蚀,水道底蚀现象。地震属性分析表明三维工区西南部残丘间水道由砂泥岩互层充填,形成长条形强振幅,而残丘为中-低振幅;地震、钻井资料分析表明丘形反射（残丘）由钙质泥岩和泥岩组成,属于半深海沉积,且残丘内部波阻抗为5.0×106~6.5×106kg/m3·m/s,低于火山岩、灰岩波阻抗,属于砂泥岩地层范畴;根据梅山组下段水道由西向东强振幅变弱、分叉、前积反射和海山附近底流（等深流）沉积剥蚀特征综合判定底流古流向自西向东,根据海山两翼地震反射特征推测底流可追溯至晚中新世早期（11.6 Ma BP）,综合分析认为,研究区中中新统梅山组丘形反射是晚中新世早期底流切割梅山组地层形成的残丘。     

Petroleum retention in the Mandal Formation,Central Graben,Norway

Upper Jurassic organic matter-rich, marine shales of the Mandal Formation have charged major petroleum accumulations in the North Sea Central Graben including the giant Ekofisk field which straddles the graben axis. Recent exploration of marginal basin positions such as the Mandal High area or the Søgne Basin has been less successful, raising the question as to whether charging is an issue, possibly related to high thermal stability of the source organic matter or delayed expulsion from source to carrier.The Mandal Formation is in part a very prolific source rock containing mainly Type II organic matter with <12 wt.-% TOC and HI < 645 mg HC/g TOC but Type III-influenced organofacies are also present. The formation is therefore to varying degrees heterogeneous. Here we show, using geochemical mass balance modelling, that the petroleum expulsion efficiency of the Mandal Formation is relatively low as compared to the Upper Jurassic Draupne Formation, the major source rock in the Viking Graben system. Using maturity series of different initial source quality from structurally distinct regions and encompassing depositional environments from proximal to distal facies, we have examined the relationship between free hydrocarbon retention and organic matter structure. The aromaticity of the original and matured petroleum precursors in the Mandal source rock plays a major role in its gas retention capacity as cross-linked monoaromatic rings act on the outer surface of kerogen as sorptive sites. However, oil retention is a function of both kerogen and involatile bitumen compositions. Slight variations in total petroleum retention capacities within the same kerogen yields suggest that texture of organic matter (e.g. organic porosity) could play a role as well.     

Pressure cells and pressure seals in the UK Central Graben

The Central Graben of the North Sea is characterised by high levels of overpressure (up to 40 MPa overpressure at 4500 m depth). We present pressure data for Cenozoic and Mesozoic reservoirs. Palaeocene sandstones control pressures in Tertiary mudstones and Cretaceous Chalk by acting as a regional ‘drain’. We divide the Jurassic into 18 pressure cells. The rift structure of the Graben controls the magnitude of pressure in each cell. Lateral hydraulic communication exists over 10 km distance between deeply-buried terraces (> 5000 m depth) and shallow structural highs (< 4500 m depth). Lateral communication increases pressure in the structurally-elevated sandstones to the minimum stress. This dynamic process produces zones of vertical fluid flow on the Forties-Montrose High, termed Leak Points. Vertical flow at Leak Points produces a 20 MWm⁻² heat flow anomaly and controls hydrocarbon retention. Leak Points are water-wet, while deep terraces in hydraulic communication with Leak Points are condensate-bearing. The Kimmeridge Clay Fm. forms the pressure seal in deep terraces.     

Garzón Massif basement tectonics: Structural control on evolution of petroleum systems in upper Magdalena and Putumayo basins,Colombia

The Garzón Massif, is an active Laramide style basement uplift flanked by the Upper Magdalena Valley (UMV) and the Putumayo Basin. In this paper we use new gravity, magnetic, well and seismic data for the first geophysical interpretation of the Garzón Massif. The Garzón/Algeciras fault has been previously interpreted as a right-lateral strike-slip fault. The new seismic, well, and gravity data demonstrates that the Garzón fault is also a low-angle (12–17°) Andean age fault thrusting PreCambrian basement 10–17 km northwestward over Miocene sediments of the UMV in a prospective footwall anticline.The new geophysical data as well as previous field mapping were used to produce the first gravity and magnetic maps and retrodeformable structural cross section of the northern Garzón Massif. The new model distinguishes for the first time distinct episodes of “thin-skinned” and “thick-skinned” deformation in the Garzón Massif. The model indicates approximately 43 km of Early to Middle Miocene shortening by “thin-skinned” imbricate thrusting contemporaneous with the uplift of the nearby southern Central Cordillera (∼9–16 Ma) and the main hydrocarbon expulsion event for the UMV and Putumayo Basin. This was followed by at least 22 km of Late Miocene (3–6 Ma) “thick-skinned” Andean shortening and 7 km of uplift on the symmetrical Garzón thrust and a SE-verging basement thrust fault zone. The Andean uplift interrupted and exposed the hydrocarbon migration pathways to the Putumayo Basin.3-D volume fracture analysis was used for the first time in this paper together with the first seismic and well data published for the Topoyaco and Miraflor structures to test closure models for the Topoyaco foothills. Intense fracturing is observed in the Topoyaco basement monocline from the near-surface to depths of over 3.5 km. The high level of fracturing permitted freshwater flushing and oil biodegradation and hydrocarbon escape. In contrast, the Miraflor-1 well, located just southwest of the Topoyaco block, tested light gravity oil and is sealed from groundwater flushing and biodegradation by a backthrust.     

Seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the Qiongdongnan Basin,northern South China Sea

Our analysis of approximately 40,000 km of multichannel 2-D seismic data, reef oil-field seismic data, and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas (water depth ≥ 500 m) of the Qiongdongnan Basin (QDNB), northern South China Sea. High-resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid-Miocene Meishan Formation in the Beijiao and Ledong–Lingshui Depressions developed reef carbonates. The seismic features of the reef carbonates in these two areas include chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies, similar to the seismic characteristics of the LH11-1 reef reservoir in the Dongsha Uplift and Island Reef of the Salawati Basin, Indonesia, which house large oil fields. The impedance values of reefs in the Beijiao and Ledong–Lingshui Depressions are 8000–9000 g/cc × m/s. Impedance sections reveal that the impedance of the LH11-1 reef reservoir in the northern South China Sea is 8000–10000 g/cc × m/s, whereas that of pure limestone in BD23-1-1 is >10000 g/cc × m/s. The mid-Miocene paleogeography of the Beijiao Depression was dominated by offshore and neritic environments, with only part of the southern Beijiao uplift emergent at that time. The input of terrigenous sediments was relatively minor in this area, meaning that terrigenous source areas were insignificant in terms of the Beijiao Depression; reef carbonates were probably widely distributed throughout the depression, as with the Ledong–Lingshui Depression. The combined geological and geophysical data indicate that shelf margin atolls were well developed in the Beijiao Depression, as in the Ledong–Lingshui Depression where small-scale patch or pinnacle reefs developed. These reef carbonates are promising reservoirs, representing important targets for deepwater hydrocarbon exploration.     

Sedimentary and structural evolution of the Eastern South Korea Plateau (ESKP), East Sea (Japan Sea)

The East Sea (Japan Sea) is a semi-enclosed back-arc basin that is thought to preserve a significant record of tectonic evolution and paleo-climatic changes of Eastern Asia during the Neogene. We use here 2-D regional multi-channel seismic reflection profiles and borehole data from Expedition 346 of the Integrated Ocean Drilling Program (IODP) to provide new constraints on the geological history of the Eastern South Korea Plateau (ESKP). The ESKP represents a structurally-complex basement high in the southwestern East Sea which formed during rifting of the back-arc basin. Our new observations show that the ESKP is composed of numerous horsts and grabens controlled by NE-trending normal faults. The acoustic basement is blanketed by Oligocene to recent sediments that have preferentially accumulated in topographic lows (up to 1.5 km thick) and have been cored during Expedition 346 at Site U1430 close to the southern margin of the ESKP. Seismic profiles in the ESKP reveal three units separated by regional unconformities. These seismic units closely correspond to IODP lithostratigraphic units defined at Site U1430, where biostratigraphic data can be used to constrain the timing of three main evolutionary stages of the ESKP. Stage 1 was related to rifting in the late Oligocene and middle Miocene, terminated by a regional uplift leading to an erosional phase in the middle Miocene. Stage 2 was associated with subsidence in the middle and late Miocene and uplift and accompanying erosion or non-deposition in the latest late Miocene. Stage 3 (Pliocene to present) recorded overall uniform hemipelagic-pelagic subsidence of the ESKP with short-lived tectonically-induced uplifts in the late middle Miocene and latest Miocene-early Pliocene. The three stages of evolution of the ESKP closely correlate to sedimentary changes since the Oligocene and suggest a direct control of regional/local tectonics on sedimentation patterns in the southwestern East Sea, with secondary influence of regional climatic and paleo-oceanographic processes.     

Morphology,sedimentary infill and depositional environments of the Early Quaternary North Sea Basin (56°–62°N)

The North Sea Basin has been subsiding during the Quaternary and contains hundreds of metres of fill. Seismic surveys (170 000 km²) provide new evidence on Early Quaternary sedimentation, from about 2.75 Ma to around the Brunhes-Matuyama boundary (0.78 Ma). We present an informal seismic stratigraphy for the Early Quaternary of the North Sea, and calculate sediment volumes for major units. Early Quaternary sediment thickness is > 1000 m in the northern basin and >700 m in the central basin (total about 40 000 km³). Northern North Sea basin-fill comprises several clinoform units, prograding westward over 60 000 km². Architecture of the central basin also comprises clinoforms, building from the southeast. To the west, an acoustically layered and mounded unit (Unit Z) was deposited. Remaining accommodation space was filled with fine-grained sediments of two Central Basin units. Above these units, an Upper Regional Unconformity-equivalent (URU) records a conformable surface with flat-lying units that indicate stronger direct glacial influence than on the sediments below. On the North Sea Plateau north of 59°N, the Upper Regional Unconformity (URU) is defined by a shift from westward to eastward dipping seismic reflectors, recording a major change in sedimentation, with the Shetland Platform becoming a significant source. A model of Early Quaternary sediment delivery to the North Sea shows sources from the Scandinavian ice sheet and major European rivers. Clinoforms prograding west in the northern North Sea Basin, representing glacigenic debris flows, indicate an ice sheet on the western Scandinavian margin. In the central basin, sediments are generally fine-grained, suggesting a distal fluvial or glacifluvial origin from European rivers. Ploughmarks also demonstrate that icebergs, derived from an ice sheet to the north, drifted into the central North Sea Basin. By contrast, sediments and glacial landforms above the URU provide evidence for the later presence of a grounded ice sheet.     

Late Devonian rifting in the central North Sea: Evidence from altered felsic volcanic rocks in the Embla oil field

In the Embla oil field on the northern flank of the Mid North Sea High, the central North Sea, multiple quartz porphyric volcanic beds at ca. 4600 m depth form part of a volcano-sedimentary interval above the Caledonian basement as interpreted from seismic data. Zircon U–Pb laser ablation ICPMS date one bed to 374 ± 3 Ma, indicating that the volcanic rocks and interbedded sediments are early Famennian and correlate to the Buchan Formation. The volcanic rocks have been extensively clay and carbonate altered in a near-surface environment, but high field strength element data show that the protoliths were alkali rhyolites, yielding intra-plate signatures in tectonic discrimination diagrams. Famennian quartz porphyric volcanic rocks have also been reported from well A17-1 on the southern flank of the Mid North Sea High. The Famennian volcanism on the northern and southern flanks testify to an active magmatic environment in the central North Sea in the early Famennian, supporting the existence of a late Devonian proto-Central Graben rift extending northwards into the central North Sea. The rift is likely an early example of strain localisation to a zone of reduced crustal strength along the Caledonian suture between Avalonia and Baltica.     

Anisotropic models to account for large borehole washouts to estimate gas hydrate saturations in the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Alaminos Canyon 21 B well

Through the use of 3-D seismic amplitude mapping, several gas hydrate prospects were identified in the Alaminos Canyon (AC) area of the Gulf of Mexico. Two locations were drilled as part of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) in May of 2009 and a comprehensive set of logging-while-drilling (LWD) logs were acquired at each well site. LWD logs indicated that resistivity in the range of ∼2 ohm-m and P-wave velocity in the range of ∼1.9 km/s were measured in the target sand interval between 515 and 645 feet below sea floor. These values were slightly elevated relative to those measured in the sediment above and below the target sand. However, the initial well log analysis was inconclusive regarding the presence of gas hydrate in the logged sand interval, mainly because large washouts caused by drilling in the target interval degraded confidence in the well log measurements. To assess gas hydrate saturations in the sedimentary section drilled in the Alaminos Canyon 21 B (AC21-B) well, a method of compensating for the effect of washouts on the resistivity and acoustic velocities was developed. The proposed method models the washed-out portion of the borehole as a vertical layer filled with sea water (drilling fluid) and the apparent anisotropic resistivity and velocities caused by a vertical layer are used to correct the measured log values. By incorporating the conventional marine seismic data into the well log analysis, the average gas hydrate saturation in the target sand section in the AC21-B well can be constrained to the range of 8–28%, with 20% being our best estimate.     

Cross-section restoration and one-dimensional basin modeling of the Central Subbasin in the southern Kunsan Basin,Yellow Sea

The petroleum system of the Kunsan Basin in the Northern South Yellow Sea Basin is not well known, compared to other continental rift basins in the Yellow Sea, despite its substantial hydrocarbon potential. Restoration of two depth-converted seismic profiles across the Central Subbasin in the southern Kunsan Basin shows that extension was interrupted by inversions in the Late Oligocene-Middle Miocene that created anticlinal structures. One-dimensional basin modeling of the IIH-1Xa well suggests that hydrocarbon expulsion in the northeastern margin of the depocenter of the Central Subbasin peaked in the Early Oligocene, predating the inversions. Hydrocarbon generation at the dummy well location in the depocenter of the subbasin began in the Late Paleocene. Most source rocks in the depocenter passed the main expulsion phase except for the shallowest source rocks. Hydrocarbons generated from the depocenter are likely to have migrated southward toward the anticlinal structure and faults away from the traps along the northern and northeastern margins of the depocenter because the basin-fill strata are dipping north. Faulting that continued during the rift phase (∼ Middle Miocene) of the subbasin probably acted as conduits for the escape of hydrocarbons. Thus, the anticlinal structure and associated faults to the south of the dummy well may trap hydrocarbons that have been charged from the shallow source rocks in the depocenter since the Middle Miocene.     

Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin,East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.     

Distribution of the Neogene Utsira Sand and the succeeding deposits in the Viking Graben area, North Sea

The sandy quartzose parts of the Utsira Formation, the Middle Miocene to mid Pliocene Utsira Sand, extends north–south along the Viking Graben near the UK/Norwegian median line for more than 450 km and 75–130 km east–west. The Utsira Sand is located in basin-restricted seismic depocentres, east of and below prograding sandy units from the Shetland Platform area with Hutton Sands. The Utsira Sand reaches thicknesses up to ca. 300 m in the southern depocentre and 200 m in the two northern depocentres with sedimentation rates up to 2–4 cm/ka. Succeeding Plio–Pleistocene is divided into seismic units, including Base Upper Pliocene, Shale Drape, Prograding Complex and Pleistocene. The units mainly consist of clay, but locally minor sands occur, especially at toes of prograding clinoforms (bottom-set sands) and in the Pleistocene parts, and the total thickness covering the Utsira Sand is in most places more than 800 m, but thins towards the margins.     

Seismic stratigraphy of the Adare Trough area, Antarctica

The Adare Trough, located 100 km NE of Cape Adare, Antarctica, is the extinct third arm of a Tertiary spreading ridge that separated East from West Antarctica. We use seismic reflection data, tied to DSDP Site 274, to link our seismic stratigraphic interpretation to changes in ocean-bottom currents, Ross Sea ice cover, and regional tectonics through time. Two extended unconformities are observed in the seismic profiles. We suggest that the earliest hiatus (early Oligocene to Mid-Miocene) is related to low sediment supply from the adjacent Ross Shelf, comprised of small, isolated basins. The later hiatus (mid-Miocene to late Miocene) is likely caused by strong bottom currents sourced from the open-marine Ross Sea due to increased Antarctic glaciation induced by mid-Miocene cooling (from Mi-3). Further global cooling during the Pliocene, causing changes in global ocean circulation patterns, correlates with Adare Basin sediments and indicate the continuing but weakened influence of bottom currents. The contourite/turbidite pattern present in the Adare Trough seismic data is consistent with the 3-phase contourite growth system proposed for the Weddell Sea and Antarctic Peninsula. Multibeam bathymetry and seismic reflection profiles show ubiquitous volcanic cones and intrusions throughout the Adare Basin that we interpret to have formed from the Oligocene to the present. Seismic reflection profiles reveal trans-tensional/strike-slip faults that indicate oblique extension dominated Adare Trough tectonics at 32–15 Ma. Observed volcanism patterns and anomalously shallow basement depth in the Adare Trough area are most likely caused by mantle upwelling, an explanation supported by mantle density reconstructions, which show anomalously hot mantle beneath the Adare Trough area forming in the Late Tertiary.     

Crustal structure and tectonic provinces of the Riiser-Larsen Sea area (East Antarctica): results of geophysical studies

About 16,000 km of multichannel seismic (MCS), gravity and magnetic data and 28 sonobuoys were acquired in the Riiser-Larsen Sea Basin and across the Gunnerus and Astrid Ridges, to study their crustal structure. The study area has contrasting basement morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the Riiser-Larsen Sea shelf, 26–28 km under the Gunnerus Ridge, 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-km-wide block with increased density and magnetization is modeled from potential field data in the upper crust of the inshore zone and is interpreted as associated with emplacement of mafic intrusions into the continental margin of the southern Riiser-Larsen Sea. In addition to previously mapped seafloor spreading magnetic anomalies in the western Riiser-Larsen Sea, a linear succession from M2 to M16 is identified in the eastern Riiser-Larsen Sea. In the southwestern Riiser-Larsen Sea, a symmetric succession from M24B to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by younger lineation M22–M22n. It is proposed that seafloor spreading stopped at about M23 time and reoriented to the M22 opening direction. The seismic stratigraphy model of the Riiser-Larsen Sea includes five reflecting horizons that bound six seismic units. Ages of seismic units are determined from onlap geometry to magnetically dated oceanic basement and from tracing horizons to other parts of the southern Indian Ocean. The seaward edge of stretched and attenuated continental crust in the southern Riiser-Larsen Sea and the landward edge of unequivocal oceanic crust are mapped based on structural and geophysical characteristics. In the eastern Riiser-Larsen Sea the boundary between oceanic and stretched continental crust is better defined and is interpreted as a strike-slip fault lying along a sheared margin.     

Morpho-sedimentary evidence for a canyon�Cchannel�Ctrench interconnection along the Taiwan�CLuzon plate margin, South China Sea

Examining bathymetric and seismic reflection data collected from the deep-sea region between Taiwan and Luzon in 2006 and 2008, we identified a connection between a submarine canyon, a deep-sea channel, and an oceanic trench in the northern South China Sea. The seafloor of the South China Sea north of 21°N is characterized by two broad slopes: the South China Sea Slope to the west, and the Kaoping Slope to the east, intersected by the prominent Penghu Canyon. This negative relief axis parallels the strike of the Taiwan orogen, extends downslope in an approx. N–S direction, and eventually merges with the northern Manila Trench via a hitherto unidentified channel. The discovery of this channel is pivotal, because it allows connecting the Penghu Canyon to the Manila Trench. This channel is 80 km long and 20–30 km wide, with water depths of 3,500–4,000 m. The progressive morphological changes recorded in the aligned canyon, channel, and trench suggest that they represent three distinct segments of the same longitudinal sediment conduit from southern Taiwan to the northern Manila Trench. Major sediment input would be via the Kaoping Canyon and Kaoping Slope, with a smaller contribution from the South China Sea Slope. We determined the northern end of the Manila Trench to be located at about 20°15′N, 120°15′E, where sediment accumulation has produced a bathymetry shallower than 4,000 m, thereby abruptly terminating the trench morphology. Comparison with existing data reveals a similarity with, for example, the Papua New Guinea–Solomon Sea Plate convergent zone, another modern analog of a mountain source to oceanic sink longitudinal sediment transport system comprising canyon–channel–trench interconnections.     

The Central Submarine Canyon in the Qiongdongnan Basin, northwestern South China Sea: Architecture, sequence stratigraphy, and depositional processes

Submarine canyons have been the subject of intense studies in recent years because of their close link to deepwater systems. The Central Canyon is a large unusual submarine canyon in the northwestern margin of the South China Sea, has a total length of about 425 km and is oriented sub-parallel to the continental slope. Using integrated 2D/3D seismic, well log, core, and biostratigraphy data, the current study documents the stratigraphic framework, internal architecture, depositional processes, and controlling factors of the segment of the Central Canyon located in the Qiongdongnan Basin.The integrated analysis shows that the canyon fill consists of four 3^rd-order sequences, SQ4, SQ3, SQ2, and SQ1. Each of them is bounded by regionally important erosional surfaces (3^rd-order sequence boundaries). Within each 3^rd-order sequence there is maximum regressive surface separating a regressive systems tract in the lower part and a transgressive systems tract in the upper part. Nine facies are identified and are further grouped into five depositional units, DU1 through DU5.The canyon evolved through four cut-and-fill stages, with a change from predominantly axial cut-and-fill to primarily side cut-and-fill. Axial cut-and-fill dominated during the first stage, and the slope-subparallel paleo Xisha Trough was intensely eroded by large-scale axial gravity flows. During the second cut-and-fill stage, the Central Canyon experienced both axial and side cut-and-fill. The third stage was dominated by side cut-and-fill. The canyon was eroded and fed by slope channels that transported sandy sediments from the shelf to the north during regression, and was covered by side-derived muddy MTCs during transgression. The last stage was also dominated by side cut-and-fill. The canyon, however, was filled predominantly by side-derived muddy MTCs.Evolution and depositional processes in the Central Canyon were likely controlled by slope-subparallel negative-relief induced by paleo-seafloor morphology, structural inversion of the Red River Fault and the slope-subparallel basement faults. Additionally, Coriolis force, sea-level fluctuations, high sedimentation rate, and rapid progradation of the slope also controlled and influenced the depositional processes, and internal architectures of the canyon.     

Characteristics and origins of middle Miocene mounds and channels in the northern South China Sea

Numerous elongated mounds and channels were found at the top of the middle Miocene strata using 2D/3D seismic data in the Liwan Sag of Zhujiang River Mouth Basin(ZRMB) and the Beijiao Sag of Qiongdongnan Basin(QDNB). They occur at intervals and are rarely revealed by drilling wells in the deepwater areas. Origins of the mounds and channels are controversial and poorly understood. Based on an integrated analysis of the seismic attribute, palaeotectonics and palaeogeography, and drilling well enco...