Seaward-dipping Reflectors Research Articles

Seaward dipping reflectors along the SW continental margin of India: Evidence for volcanic passive margin

Multi-channel seismic reflection profiles across the southwest continental margin of India (SWCMI) show presence of westerly dipping seismic reflectors beneath sedimentary strata along the western flank of the Laccadive Ridge — northernmost part of the Chagos-Laccadive Ridge system. Velocity structure, seismic character, 2D gravity model and geographic locations of the dipping reflectors suggest that these reflectors are volcanic in origin, which are interpreted as Seaward Dipping Reflectors (SDRs).The SDRs; 15 to 27 km wide overlain by ∼1 km thick sediment; are observed at three locations and characterized by stack of laterally continuous, divergent and off-lapping reflectors. Occurrence of SDRs along western flank of the Laccadive Ridge adjacent to oceanic crust of the Arabian Basin and 2D crustal model deduced from free-air gravity anomaly suggest that they are genetically related to incipient volcanism during separation of Madagascar from India.We suggest that (i) SWCMI is a volcanic passive margin developed during India-Madagascar breakup in the Late Cretaceous, and (ii) continent-ocean transition lies at western margin of the Laccadive Ridge, west of feather edge of the SDRs. Occurrence of SDRs on western flank of the Laccadive Ridge and inferred zone of transition from continent to ocean further suggest continental nature of crust of the Laccadive Ridge.

Variability in the crustal structure of the West Indian Continental Margin in the Northern Arabian Sea

ABSTRACT In this case study of the West Indian Continental Margin we present an interpretation of a volcanic margin structure based on regional mapping of high quality 2D seismic data in conjunction with regional satellite derived gravity data and selected subsidence analyses. The area shows many classic characteristics of a volcanic-type margin. Volcanic facies identified and mapped along the margin include seaward-dipping reflectors (SDRs), sub-aerial seamounts, clinoform packages interpreted as lava and volcaniclastic delta systems and thick, seismically layered sequences interpreted as volcanically derived sediment deposited in both fluvial and marine environments. The results show major variation in the overall thickness and style of volcanism across the margin both in dip and strike directions which may be related to variability in influence of the Deccan Plume in addition to localization along structural features inherited from older tectonic events. Our interpretation of rapid lateral variation in the thickness of extrusive volcanism has important implications for the distribution, preservation and hydrocarbon potential of the pre-rift sequence across the margin. The interpreted crustal structure also has a major impact on predictions of the historical and present-day heat-flow into the post-rift section. Our interpretation of the timing and distribution of volcanism is consistent with the presence of a broad region of elevated mantle potential temperatures at the time of the final break-up event on the West Indian Continental Margin, commonly attributed to the Deccan/Reunion Plume. Pre-existing structural heterogeneities appear to have played an important part in controlling the distribution of volcanism. Interpreted tectonic subsidence, based on backstripping analysis of the post-break-up interval, is also shown to be consistent with post-break-up thermal subsidence in combination with dynamic support associated with the elevated mantle temperatures into the Early Eocene.

Ground-Penetrating Radar Study of North Padre Island: Implications for Barrier Island Internal Architecture, Model for Growth of Progradational Microtidal Barrier Islands, and Gulf of Mexico Sea-Level Cyclicity

Abstract Padre Island is the widest and longest barrier island in the world. It is 200 km long, up to 3 km wide, and 10–15 m thick. This 2,000-year-old barrier island was formed predominantly from northward longshore-drifted sediments from the Rio Grande River Delta. A 3-km-long, shore-normal, 50 MHz ground-penetrating radar (GPR) profile was collected across Padre Island from the Gulf of Mexico to Laguna Madre at the northern end of the Padre Island National Seashore. A 1-km-long, shore-parallel GPR profile was also collected and intersects the shore-parallel profile approximately 1 km landward of the Gulf of Mexico. Vibracores taken along the shore-normal profile, at the beach foreshore, in the center of the island, at the lagoon margin, and at a recent washover fan were used, in conjunction with the description of a deep rotary core taken through the northern part of the island, to constrain the GPR interpretations. In the shore-normal profile, GPR reflectors indicate both seaward- and landward-dipping clinoforms, as well as concave-upward scour-and-fill features. Seaward-dipping reflectors indicate approximately 2 km of progradation of the barrier island, suggesting an average progradation rate of 1 m/year. Landward-dipping reflectors, in the landward 1 km of the GPR profile, indicate approximately 1 km of landward growth, suggesting an average landward growth rate of 0.5 m/year. Nested concave-upward scour-and-fill features in the central part of the island are interpreted as tidal and washover channels 3–8 m deep. In strike cross section these scours are channel-form in outline and contain nearly horizontal, to slightly inclined, radar reflectors. The presence of channel-form scours supports the interpretation that landward growth was dominantly due to deposition by short-lived tidal deltas and storm washovers. Even though there was periodic landward island growth, North Padre Island has been progradational throughout its history due to a high sediment supply. The understanding of the architecture and growth history of Padre Island provides much needed information about the internal geometry and the evolution of modern barrier-island systems. In addition, systematic variations in scour depths across the island suggest that storm frequency and intensity varied during the island’s history and provides a proxy for estimating sea-level fluctuations.

Birth of a volcanic margin off Argentina, South Atlantic

Based on a dense grid of multichannel reflection seismic lines we study the evolution of a volcanic rifted margin segment off Argentina. The segment under study is located between 44°S and 41°S. We describe in detail the facies of the extrusive basaltic complexes across the margin. These include single and multiple Inner Seaward Dipping Reflectors (SDRs) with varying architecture along the margin, Outer Highs, and Outer SDR wedges. A detailed interpretation of a transitional zone between the Inner and Outer SDR wedges is presented. Multiple Inner SDRs are concentrated at the southern part of the margin segment, while in the north only one steeply dipping wedge is present. The dip of the Inner SDR wedges changes along the margin, and we propose subsidence by loading as main cause. The transitional zone between the Inner and Outer SDR wedges becomes narrower toward the north. By correlating with magnetic data we conclude that the emplacement of the 30–100 km wide multiple Inner SDRs occurred episodically. We propose an injection center which migrated in a seaward direction with proceeding extension as the origin for the multiple Inner SDRs. A scissor‐like opening of the margin segment resulted in different amounts of stretching along‐strike of the margin segment and is likely the cause for the varying distribution of the extrusive basaltic complexes. The varying extension rates within the margin segment contribute to decreasing volumes of melts generated in a northward direction within this rift segment.

The SW African volcanic rifted margin and the initiation of the Walvis Ridge, South Atlantic

The continental margin of SW Africa is typical of a volcanic rifted margin associated with a hotspot trail characterized by a large volcanic ridge, the Walvis Ridge, defining the hotspot migration, and extensive extrusive volcanism that produced seaward-dipping reflectors (SDR). Previously unpublished seismic data show two significant anomalies of the SW African Margin when compared to other typical volcanic rifted margins: (1) Hyaloclastitic outer highs are rare, and (2) the SDR in the North dip towards the Walvis Ridge. We explain these anomalies by a major transform segment close to the centre of volcanism combined with pulsed volcanism. The Walvis Ridge represents an east-west striking extrusive centre which produced a SDR sequence. Following break-up the northern boundary of the Walvis Ridge became a left lateral transform fault. Our data support the idea that a transform fault system interacting with a ridge jump were responsible for the accretion of the Sao Paulo Plateau to the American plate.

Influence of margin segmentation upon the break-up of the Hatton Bank rifted margin, NE Atlantic

The Hatton Bank margin, flanking the Iceland Basin, is an example of a volcanic rifted margin and has been studied to examine the along margin tectono-magmatic variability. Integration of 5660 km of new seismic reflection profiles with > 60,000 km 2 of new multibeam bathymetry has allowed the margin to be divided into three segments, each of which are flanked by oceanic crust. The southernmost segment is characterised by a series of inner and outer seaward dipping reflector (SDR) packages, which are separated by an “Outer High” feature. The outer SDRs are truncated by Endymion Spur, a chain of steep sided, volcanic cones connected by narrow septa or necks. The central segment has no Inner SDR package and is characterised by the presence of a continental block, the Hatton Bank Block (HBB). The northern segment is adjacent to Lousy Bank, with a wider region of SDRs recognised than to the south, and characterised by many volcanic cones. The variations in the distribution of the SDRs along the margin, the presence of the HBB and Endymion Spur all suggest that the break-up process was not a uniform smooth process along-strike. Structural segmentation controlled the variations along the margin with break-up initiated in the south, producing the SDR packages. The HBB prompted the focus of break-up to relocate outboard of the block. The northern segment was closest to the Iceland “hot-spot”, and regular seafloor spreading did not become established until Chron 21. Shortly after break-up, the eruption of Endymion Spur occurred and may have been triggered by the passage of a pulse of hot asthenospheric material along the margin. The margin segmentation pattern we describe controlled the location of the enhanced volcanism along the Endymion Spur to the southern sector. In addition the segmentation has influenced the break-up style (presence or absence of SDR) and also the location and nature of post break-up volcanism.

Age of Seychelles–India break-up

Many continental flood basalt provinces are spatially and temporally linked with continental break-up. Establishing the relative timing of the two events is a key step in determining their causal relationship. Here we investigate the example of the Deccan Traps and the separation of India and the Seychelles. Whilst there has been a growing consensus as to the age of the main phase of the Deccan emplacement (65.5 ± 1 Ma, chron 29r), the age of the rifting has remained unclear. We resolve this issue through detailed seafloor magnetic anomaly modeling (supported by wide-angle and reflection seismic results) of the north Seychelles and conjugate Laxmi Ridge/Gop Rift margins, and geochemistry and 40Ar/ 39Ar geochronology of rocks from the north Seychelles margin. We show that syn-rift volcanics offshore the Seychelles Islands in the form of seaward-dipping reflectors were most likely erupted during chron 28n, and the first organized seafloor spreading at the Carlsberg Ridge also initiated during this chron at 63.4 Ma. The severing of the Seychelles occurred by a south-eastward ridge propagation that was completed by the start of chron 27n (~ 62 Ma). A brief, pre-28r phase of seafloor spreading occurred in the Gop Rift, possibly as early as 31r–32n (~ 71 Ma). Initial extension at the margin therefore preceded or was contemporaneous with the Deccan emplacement, and separation of the Seychelles was achieved less than 3.5 Ma afterwards. This is the shortest time interval between flood basalt emplacement and break-up yet reported for any continental flood basalt-rifted margin pair. A contributing factor to the apparently short interval in the Deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere. However, we conclude that external plate-boundary forces, rather than the impact of a mantle plume, were largely responsible for the rifting of the Seychelles from India.

Geophysical insights and early spreading history in the vicinity of the Jan Mayen Fracture Zone, Norwegian–Greenland Sea

Using a new high-resolution aeromagnetic survey (JAS-05) that was acquired along the trend of the Jan Mayen Fracture Zone (JMFZ), west of the Vøring volcanic margin, we investigated the geodynamic framework of the early spreading evolution of the Norwegian–Greenland Sea. The tectonic structure, main faults and magnetic chrons have been reinterpreted based on new magnetic gridded data and integrated with bathymetry, gravity and seismic data. The new interpretation reveals more details about the early spreading history of the Norwegian–Greenland Sea in the vicinity of the JMFZ. Although anomalous melt production (seaward-dipping reflectors, underplating) associated with the breakup of the Mid-Norwegian margin has been described in many studies, we present data that suggest that significant magmatism continued episodically during the opening of the Norwegian–Greenland Sea along the trend of the JMFZ. The Vøring Spur (VS), an anomalous oceanic high, lying north of the eastern segment of the JMFZ exhibits a contrasting Bouguer gravity low and a complex magnetic signature. The gravity signature of the VS can be modelled and explained as an abnormal thick oceanic crust, which locally can reach up to 15 km. We propose that the thick oceanic crust (overcrusting) was syn-rift and formed during Mid- to Late Eocene. A plate reconstruction at Eocene time suggests that the VS could be part of a triple junction initiated during the breakup between the Vøring Marginal High and the Greenland part of the Traill–Vøring igneous complex, now located offshore East Greenland. Mantle upwelling beneath the early spreading ridge and/or local stress reorganisation could have induced transtension and lithospheric thinning along the JMFZ and magmatic activity would have increased locally along this ‘leaky transform’. We suggest that the Early Tertiary tectono-magmatic processes that operated in the Norwegian–Greenland Sea are similar to the processes involved in the modern triple junction evolution of the Azores Plateau region.

Tectonotype of volcanic passive margins in the Norwegian-Greenland region

A tectonotype of volcanic passive margins exemplified in the conjugate Norwegian and East Greenland margins is considered, with discussion of the Paleogene igneous complexes and the regional rift structure before continental breakup. Fragments of asymmetrical rift have been retained on both sides of the ocean. Large Cretaceous pre-rift sedimentation basins marking the initial stage of the ocean opening are included into the passive margin as well. The continental breakup was accompanied by intense basaltic magmatism over a short time span. This magmatic episode was distinguished by (1) the formation of widespread plateau-basalt complexes on continents and in near-shore areas of the ocean; (2) the development of thick lava series that are recorded in seaward dipping reflector wedges; (3) thick high-velocity lower crust, resulting from magmatic underplating; (4) asymmetrical accretion of the crust and structure formation. The discussion is based on published seismic data and reference sections selected for each margin with consideration of the composition and thickness of the igneous rocks, their lateral variations, source composition, and eruption and crust formation conditions. The characteristic feature of both sections is the two-member structure of volcanic complexes with substantial geochemical differences between the rocks from the lower and upper parts of the section, which correspond to the pre-breakup and breakup phases. At the initial phase, small magma volumes were melted out from the lithosphere. The geochemical signatures of the upper parts of the sections testify to the melting of the asthenospheric mantle. Their spatiotemporal variations reflect the ascent and melting of the deep plume, which was active during and after continental breakup. In the Greenland area, near the central part of the plume, a N-MORB-type mantle magma source gave way to a depleted Iceland-type mantle, while apart from the central part of the plume, its effect is expressed only in the enormous volume of mantle-derived melt without migration of its source. A variety of evidence is provided for the plume’s activity: the great thickness of the volcanic complexes and the relatively stable composition of the melt; the elevated temperature in the mantle; the specific geochemistry of the breakup-related lavas and their lateral zoning; conclusions on the necessity of dynamic support of volcanic eruptions; and recent results of seismographic tomography. The continental breakup inherited a system of older sedimentary basins in the zone of prolonged extension of the lithosphere in the North Atlantic. The continuous dynamic support of extension was most likely provided by long-term ascent of the Iceland plume. The comparison of the considered tectonotype with other volcanic and non-volcanic margins opens the way to further elucidation of the geodynamic processes responsible for the ocean opening.

Architecture of volcanic rifted margins: new insights from the Exmouth – Gascoyne margin, Western Australia

Australia's western continental margin formed during Gondwana breakup between Australia – Antarctica and India, leaving behind an evolving plate boundary with significant new magmatic crust accreted to the continental fragments. In this context, the margin segment from the outer Exmouth Plateau to the Gascoyne Abyssal Plain has been repeatedly studied as an example of volcanic margin formation. However, many recent analyses based on selective interpretation of geophysical datasets have come to conflicting conclusions regarding both the tectonic architecture and the age of breakup on this margin. These differences profoundly impact on models for both the global plate circuit and for the timing and extent of magmatism implicated in volcanic breakup processes. We present new interpretations of the distribution of magmatic and pre-rift rock packages in this margin, based on the integrated interpretation of two deep crustal transects with existing seismic-reflection, refraction, gravity, magnetic and geochemical data. Interpretations are constrained by data from sparse Ocean Drilling Program and petroleum-exploration drilling, and dredging. We find evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling extensional fault-controlled basins developed in a broad volcanic-margin transition zone between the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the form of seaward-dipping reflector sequences, and are dense and magnetised. Most significantly, these packages give rise to potential-field anomalies that have previously been interpreted as due to seafloor spreading. Recognition of these packages in a volcanic-margin transition zone has implications for the recognition of the inboard edge of unequivocal oceanic crust, the oceanic – volcanic-margin boundary. The main locus of igneous activity in the volcanic-margin transition zone off the Exmouth Plateau is spatially offset from a previously recognised high-velocity zone, suggesting that these two phenomena may not be temporally related. Seismically imaged differences in total thinning and partitioning of thinning, between upper and lower crust, provide support for models of depth-dependent thinning previously proposed for this margin.

The structure of the lower crust at the Argentine continental margin, South Atlantic at 44°S

It is well established that the Argentine passive margin is of the rifted volcanic margin type. This classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope, manifested by seismic data as a seaward dipping reflector sequence (SDRS). Here, we investigate the deep structure of the Argentine volcanic margin at 44°S over 200 km from the shelf to the deep oceanic Argentine Basin. We use wide-angle reflection/refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times. The resulting P-wave velocity-depth model confirms the typical volcanic margin structure. An underplated body is resolved as distinctive high seismic velocity ( v p up to 7.5 km/s) feature in the lower crust in the prolongation of a seaward dipping reflector sequence. A remarkable result is that a second, isolated body of high seismic velocity ( v p up to 7.3 km/s) exists landward of the first high-velocity feature. The centres of both bodies are 60 km apart. The high-velocity lower-crustal bodies likely were emplaced during transient magmatic–volcanic events accompanying the late rifting and initial drifting stages. The lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the South Atlantic rift evolved by instantaneous breakup of longer continental margin segments. These segments are confined by transfer zones that acted as rift propagation barriers. A lower-crustal reflector was detected at 3 to 5 km above the modern Moho and probably represents the lower boundary of stretched continental crust. With this finding we suggest that the continent–ocean boundary is situated 70 km more seaward than in previous interpretations.

Magnetic Evidence for Volcanism at Eastern Continental Margin of India: Juxtaposition with Elan Bank (Southern Indian Ocean)

The rifted Eastern Continental Margin of India (ECMI) has evolved as a result of breakup of East Gondwanaland. Previous geophysical studies of the continental margin have not elucidated upon its volcanic nature. Magnetics plays a useful role in the study of continental margins, particularly in identifying the volcanic units. The aeromagnetic map of the offshore Mahanadi basin of ECMI displays a conspicuous linear anomaly along the continental shelf. A comprehensive study of the published aeromagnetic, marine magnetic and gravity data of the offshore Mahanadi basin reveals the existence of a seaward dipping volcanic unit in the offshore Mahanadi basin bordering the Hinge zone. This inference suggests that the ECMI is a volcanic rifted margin. The study further indicates the deepening of the basement towards the sea. In addition, the existing geological studies on the ECMI demarcated the probable limit of the continental crust by studying the basement detached tectonic style of the sedimentation in sub-surface configuration of the East coast basins of India. The probable continental crustal limit, the Hinge zone, and the inner edge of the presently inferred volcanic unit conform to one another spatially in the offshore Mahanadi region. These features characterize the inferred volcanic body as seaward dipping reflectors (SDRs) that usually occur at the rifted continental margins. The deepening of the basement towards the sea and the presence of the volcanic body on the continental margin are indicative of the transitional nature of the crust. It is generally accepted that Antarctica and India were juxtaposed before the breakup of Gondwanaland. But the microcontinents in the southern Indian Ocean are neglected in the reconstruction of Gondwanaland continents. The recent studies of the discovery of continental crust within the Elan Bank (EB) microcontinent show that the EB was contiguous with the East coast of India before the breakup of Gondwanaland. Moreover, it is reported that the upper igneous crust of the EB consists of a 2–3 km thick layer of accumulated lava flows originating from the Kerguelen hotspot. An estimate shows that the total volume of volcanic and plutonic component of the Elan Bank is about 0.3 million cubic kilometers. The present inference of a volcanic body from the offshore Mahanadi basin is in agreement with the above observations of the juxtaposition of EB with ECMI.

South Atlantic divergent margin evolution: rift-border uplift and salt tectonics in the basins of SE Brazil

Abstract The South Atlantic Ocean evolved after rupture of the São Francisco–Congo–Rio de la Plata–Kalahari cratonic landmass and the Late Proterozoic fold belts. Break-up in the South Atlantic realm developed diachronously: rifting started in the south (Argentina) during the Jurassic and progressed towards the equatorial segment. The central portion was controlled by a rift-resistant cratonic nucleus (the São Francisco–Congo craton) and as a result underwent development of narrow basins; parts controlled by Neoproterozoic fold belts developed wide basins. The final break-up of western Gondwana and the onset of plate divergence were marked by thick wedges of seaward-dipping reflectors, located near the incipient ocean-ridge spreading centre that had already been formed by the time Aptian evaporites were deposited. Subsequently, a few episodes of intraplate tectonic and magmatic activity affected the Santos, Campos and Espírito Santo basins. Post-break up development of the offshore basins was affected by gravity gliding over the Aptian evaporites. Continental uplift may be invoked as the main cause of salt mobilization, generating prograding clastic wedges that thickened basin-wards and produced a loading effect on the salt basin. Coupled with onshore erosional unloading and the effects of the gravity gliding, this probably resulted in further flexural uplift of the continental margin.

Seismic volcanostratigraphy of the Gascoyne margin, Western Australia

Integrated interpretation of seismic, gravity, and magnetic data has revealed the presence of a large breakup-related volcanic complex on the rifted Gascoyne Margin, Western Australia. Using the concept of seismic volcanostratigraphy, we have identified three main volcanic seismic facies units related to the extrusive volcanism: 1) Landward Flows, 2) Seaward Dipping Reflections (SDR), and 3) Volcanic Protrusions. Additionally, we have identified four seismic facies units related to the tectonomagmatic evolution of the margin: 4) Domes, 5) Moho, 6) Top Underplating, and 7) Sill Intrusions. The Galah Rise comprises the majority of the volcanic complex and is dominated by 100–200 km long, NE-striking volcanic ridges surrounded by multiple sets of deep-marine emplaced SDRs. An estimated excess crustal volume of the 50 000 km 2 large Galah Rise is 350 000 km 3. The volcanic complex is located at the outer corner of a shear-rift margin setting. The magmatism is sparse on the shear margin, massive near the outer corner and decreases NE-wards along the rifted margin segment and away from the fracture zone. The Gascoyne Margin is comparable to the volcanic Vøring Margin, NE Atlantic, in terms of dimensions, structural setting and distribution of the volcanic seismic facies units. However, the along-margin variations in magmatism are more prominent on the Gascoyne Margin.

Demerara Plateau - the structure and evolution of a transform passive margin

SUMMARY The Amazon Cone Experiment acquired two transects (Profile sD&A )across the Demerara Plateau, part of the French Guiana-Northeast Brazil continental margin, to better understand rift and transform-style margin evolution. Profile A images an intermediate-type margin formed as a result of trans-tensional extension. In this paper we describe the modelling of wide-angle and multichannel seismic and gravity data from Profile D, to reveal whole crustal structure and features exhibiting transform characteristics. Combining these results with other studies in the region and comparing our results with ‘young’ rift analogues, we develop a model of along-margin segmentation that explains the evolution of the west equatorial Atlantic. Interpretation of the velocity-depth model for Profile D shows a 35‐37 km thick continental crust which thins to 10‐11 km over a distance of 320 km. This thinning is accommodated in two regions. The narrowest region, associated with the ocean‐continent transition, is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break-up geometry of the Central Atlantic. Given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning, located 162 km landward of the ocean‐continent transition, is a result of rifting suborthogonal to the profile orientation. There is no evidence of rotated faulted blocks, half graben structures or riftrelated magmatism, manifest as either seaward-dipping reflectors or as a high-velocity region within the lower crust. The Demerara Plateau is, therefore, interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform-type zones in which trans-tensional extension is accommodated. In contrast to Profile A, modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift-type setting. Offshore, unusually thin oceanic crust of 3.3‐5.7 km thickness is identified which is consistent with similar observations further south. In the absence of identifiable magnetic anomalies, best estimates of the initial half-spreading rate of ∼20 mm yr −1 suggest that the thin crust throughout the region is unlikely to be a result of ultra-slow spreading but, instead, it is most likely due to a reduced magma supply at numerous, long-lived transform faults reflected by those presently offsetting the Mid-Atlantic Ridge in this equatorial setting.

Margin segmentation and volcano-tectonic architecture along the volcanic margin off Argentina/Uruguay, South Atlantic

Some 25,000 kilometers of regional multichannel seismic data, acquired by BGR along the continental margins off Argentina and Uruguay document that the Early Cretaceous South Atlantic continental breakup and initial sea-floor spreading were accompanied by large-scale, transient volcanism emplacing voluminous extrusives, manifested in the seismic data by huge wedges of seaward dipping reflectors (SDRs). The emplacement of the deeply buried, 60–120 km wide SDRs was probably episodic as documented by at least three superimposed SDRs units. Distinct along-margin variations in architecture, volume, and width of the SDRs wedges are probably related to margin segmentation. It is suggested that the margin can be divided, at least, in four compartments bounded by the Falkland Fracture Zone/Falkland transfer, the Colorado transfer, the Ventana transfer and the Salado transfer. The individual margin segments reflect in the distribution and thickness of the post-rift sediments. The individual transfer zones may have acted as rift propagation barriers, selectively directing rift segments in left stepping patterns along the western South Atlantic margin. Although we found extensive variations in the architecture, style, and extent of the seaward dipping reflector sequences, a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the SDRs wedges decreases northward within the individual margin segments. We suggest that mainly adiabatic decompression and melt generation from shallow sources can explain distinct along-margin variations in the volcano-tectonic architecture and volumes of extruded magmas.

Seismic study of the transform‐rifted margin in Davis Strait between Baffin Island (Canada) and Greenland: What happens when a plume meets a transform

The Davis Strait transform margin was studied using a 630‐km‐long wide‐angle reflection/refraction seismic transect extending from SE Baffin Island to Greenland. Dense airgun shots were recorded by 28 ocean bottom seismometers deployed along the line. A P wave velocity model was developed from forward and inverse modeling of the wide‐angle data and incorporation of coincident deep multichannel reflection seismic data. Off Baffin Island in the Saglek Basin, 7 to 11‐km‐thick two‐layered continental crust (5.8–6.6 km/s) is observed. Off Greenland, continental crust is divided into three layers (5.4–6.8 km/s) with a maximum thickness of 20 km. Farther offshore Greenland the crust thins to 7–12 km and the lower crust disappears. Between the continental blocks a 140‐km‐wide zone with oceanic crust (layer 2 is 5.4–6.2 km/s and layer 3 is 6.7–7.0 km/s) is located. The western half of this zone is interpreted to be part of a volcanic margin with seaward dipping reflectors; the eastern part is associated with the Ungava fault zone (UFZ), the major transform fault in Davis Strait. The UFZ thus acted as leaky transform fault during phases of transtension. Southward flow of material from the Iceland plume created a 4 to 8‐km‐thick underplated layer (7.4 km/s) beneath the thinned portions of the continental crust and beneath previously emplaced oceanic crust. Plume related Paleogene volcanism is indicated by an up to 4‐km thick layer (4.3–5.8 km/s) with basalts and interbedded sediments that can be traced from SE Baffin Island 400 km toward the east.

3-D Architecture and Sequence Stratigraphic Evolution of a Forced Regressive Top-Truncated Mixed-Influenced Delta, Cretaceous Wall Creek Sandstone, Wyoming, U.S.A.

Abstract Ground-penetrating radar (GPR) is used to delineate the 3-D facies architectural elements and examine the evolution of a top-truncated, forced-regressive, mixed-influenced delta front in the Cretaceous Wall Creek Member of the Frontier Formation, Wyoming. The GPR data provide a bridge between outcrop facies architecture and recently published high-resolution 2-D seismic studies of Quaternary delta systems. The GPR data were integrated with outcrops, photomosaics, cores, and GPS data. Two orthogonal grids of 2-D GPR profiles provide information on the 3-D facies architecture and stratigraphy of the deltaic deposits. Three main GPR architectural elements are identified within the delta front: (1) mouth bars are characterized by seaward-dipping reflections (foreset beds) in dip view, and mounded bidirectionally downlapping reflections in strike view; (2) shallow delta-slope channels show truncation and onlap and also show low-angle landward (northwest) dipping reflections. The mouth bars are locally interrupted by (3) sub-horizontal radar reflections that are interpreted as tidally modulated bars. Within the mouth-bar radar facies, both top-preserved (proximal) and top-truncated (distal) examples are observed. Top-preserved proximal bars show evidence of mounded bar crests and landward accretion, as is observed in the upstream end of mouth bars on the modern Atchafalaya Delta in the Gulf of Mexico. Distal bar deposits primarily comprise the top-truncated, seaward-dipping foresets. The GPR interpretation shows two laterally overlapping delta lobes, interpreted to represent autocyclic delta switching. The older lower lobe forms a fan-shaped delta lobe composed of top-truncated coalesced mouth bars. The second phase of bars is largely top-preserved, building over the older bars. The presence of thicker bedsets and upstream accretion suggests that the younger lobe was quickly abandoned, as seen in the overlying transgressive ravinement. Although marine erosion during transgression was less effective on these later-stage bar deposits, ravinement increased landward. A comparison of the architectural elements at a more regional scale in two separate Wall Creek delta lobes (Murphy Creek and Raptor Ridge, ~ 30 km apart) indicates that the bar complexes were deposited on the delta fronts as accretionary forced-regressive deposits during a relative sea-level fall; the older river-dominated deltaic deposits at the Murphy Creek site prograded southeastward (~ 124°), whereas the younger and more tidally influenced Raptor Ridge bar deposits expanded southward (~ 171°) before final abandonment and regional transgression. The most distal lowstand lobe sediments within the offlapping parasequence at Raptor Ridge are more completely preserved and also show the greatest tidal influence, compared to the more severely top-truncated Murphy Creek, which is in an otherwise more landward location. This challenges the notion that tidal facies are predominantly associated with transgressive or highstand systems tracts. Greater preservation of deltaic facies in forced-regressive and lowstand deltas, as shown here, has also been documented in Quaternary shelf-edge deltas, such as the Lagniappe in the Gulf of Mexico and the Mahakham in Kalimantan.

Margin segmentation of Baffin Bay/Davis Strait, eastern Canada based on seismic reflection and potential field data

The continental margin of Baffin Island was interpreted from a compilation of seismic reflection and potential field data. Based on these data, it was divided into volcanic and non-volcanic segments. In the northernmost area, coast-parallel grabens and continental basement highs are superseded beneath the slope by irregular basement topography. In combination with the velocities from refraction data and the lack of a magnetic slope anomaly, we propose a non-volcanic margin. In the central region, the seismic data show a limited region with a seaward dipping reflector sequence that may be intercalated with a sedimentary section. Both north and south of Cape Dyer extensive volcanic rocks are interpreted at and below seabed consistent with a volcanic margin. In the southern region, the depth to basement drops rapidly offshore. A thick prograding sedimentary wedge produces a high-amplitude long-wavelength free-air gravity anomaly that was previously confused with the continent–ocean boundary. Although some volcanic rocks are mapped in the region, this area is adjacent to the non-volcanic Labrador Sea margin and is interpreted to be of similar structure. Segmentation of the margin by major faults can explain the abrupt transitions from volcanic to non-volcanic margin segments. The continent–ocean boundary for the region is determined based on the potential field and seismic data. Along the Greenland plate in Davis Strait the continent–ocean boundary coincides with the Ungava Fault Zone. The continent–ocean boundary is used in conjunction with published poles of rotation to evaluate plate reconstructions at chrons 33n and 27n. The earlier reconstruction produces an extensive overlap of boundaries in the north. The later reconstruction shows a gap in the southern region. Thus, additional information is required to refine the reconstructions.

Continent‐ocean transition on the Vøring Plateau, NE Atlantic, derived from densely sampled ocean bottom seismometer data

A 180 km long seismic wide‐angle profile was acquired across the Vøring Plateau, NE Atlantic, using a tuned air gun array and three‐component ocean bottom seismometers deployed with ∼5 km spacing. The seismic P wave data have been modeled by ray tracing/inversion, and the model has been constrained by S wave and gravity modeling. The data and modeling have allowed us to depict the crustal structure and nature of the continent‐ocean transition (COT) in a classical volcanic margin case. The P wave velocity near the top of the main crustal layer is estimated to ∼6.0 km/s landward of the ∼25 km wide COT. The seaward increase to ∼6.5 km/s in the COT is conformable with heavily intruded continental crust within this zone. Farther seaward, the velocity increase to ∼6.9 km/s in the same layer suggests the presence of oceanic crust. The abundant magmatism landward of, and within, the COT is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions/underplating. The maximum thickness of the oceanic crust is measured to ∼23.5 km at the seaward termination of the COT.