Newfoundland Margin Research Articles

Crustal structure of the Newfoundland rifted continental margin from constrained 3-D gravity inversion

The rifting history of the Atlantic continental margin of Newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2-D seismic surveys. While informative, the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin. A 3-D gravity inversion of the free air data over the Newfoundland margin allows us to generate a 3-D density anomaly model that can be compared with the seismic results and used to gain insight into regions lacking seismic coverage. Results of the gravity inversion show good correspondence with Moho depths from seismic results. A shallowing of the Moho to 12 km depth is resolved on the shelf at the northern edge of the Grand Banks, in a region poorly sampled by other methods. Comparisons between sediment thickness and crustal thickness show deviations from local isostatic compensation in locations which correlate with faults and rifting trends. Such insights must act as constraints for future palaeoreconstructions of North Atlantic rifting.

Detrital sources and sediment delivery to the early post-rift (Albian-Cenomanian) Newfoundland Basin east of the Grand Banks: results from ODP Leg 210

Abstract The young narrow arm of the Atlantic Ocean that separated Newfoundland from Iberia in the Albian–Cenomanian accumulated a thick succession of muddy gravity flow deposits that were recovered by drilling at Ocean Drilling Program Site 1276 off the Grand Banks of Newfoundland. The sediment thickness and sand content are somewhat surprising given the number of contemporaneous shelf and slope basins that would have acted as traps for detritus shed from the Newfoundland margin. A set of individual studies was undertaken to trace the source of this detritus and to test hypotheses that the material might have been derived, in part, from the east (Iberia), the north (Flemish Cap and northern rift extensions), and/or from various sources to the west (Grand Banks, Interior Newfoundland, Avalon Uplift). By synthesizing seismic data, paleocurrent data from gravity flow deposits, sediment composition (sand detrital modes, clay mineralogy) and detrital mica ages we conclude that the likely source of the Albian–Cenomanian sediment was the Avalon Uplift, situated in the southern Grand Banks. Key pieces of evidence that lead us to this result are: thinning of the sediment package towards the north and thickening towards the southwest, the paleocurrent directions apparently suggesting a southwesterly source, the low proportion of potassium feldspar in the sandstones, and the lack of palygorskite in the shales as compared to Iberian-sourced successions. The apparent inconsistency of detrital mica ages from these deposits, suggesting an Iberian (Hercynian) source with age 270–340 Ma, is attributed to the localization of rifting along a Paleozoic continental suture zone, the Hercynian Front. This tectonic front separates distinctly different geologic units (domains) on the conjugate margins. Such a suture would have generated a significant peripheral foreland basin across the now-submerged shelf regions of Newfoundland allowing transfer of sediment from the Iberian to the Newfoundland domains. Upon Mesozoic rifting and uplift, these mica-bearing sediments from the foreland basin succession could have been reworked and shed eastward into the nascent Atlantic Ocean basin at ODP Site 1276. Alternatively, Hercynian metamorphism and deformation might have extended onto what is now the southeastern Grand Banks, so that young detrital micas could have been derived directly from the Avalon Uplift. Results from this work are relevant to an assessment of sandstone distribution in parts of the Salar Basin and the expected diagenetic overprint for sandy reservoir facies.

About this title - Imaging, Mapping and Modelling Continental Lithosphere Extension and Breakup

This book summarizes our present understanding of the formation of passive continental margins and their ocean–continent transitions. It outlines the geological, geophysical and petrological observations that characterize extensional systems, and how such observations can guide and constrain dynamic and kinematic models of continental lithosphere extension, breakup and the inception of organized sea-floor spreading. The book focuses on imaging, mapping and modelling lithospheric extensional systems, at both the regional scale using dynamic models to the local scale of individual basins using kinematic models, with an emphasis on capturing the extensional history of the Iberia and Newfoundland margins. The results from a number of other extensional regimes are presented to provide comparisons with the North Atlantic studies; these range from the Tethyan realm and the northern Red Sea to the western and southern Australian margins, the Basin and Range Province, and the Woodlark basin of Papua New Guinea. All of these field studies, combined with lessons learnt from the modelling, are used to address fundamental questions about the extreme deformation of continental lithosphere.

Imaging, mapping and modelling continental lithosphere extension and breakup: an introduction

Abstact This Special Publication is a direct outcome of a small but dedicated group of researchers who met in Pontresina, Switzerland, to review and define the fundamental observations characterizing extensional systems and their application in guiding and constraining modelling efforts and results. The various summaries of the keynote addresses give an objective overview of the state of the art in modelling lithospheric extensional systems, both from the regional scale using dynamic models to individual basins using kinematic models with an emphasis on capturing the extensional history of the Iberia and Newfoundland margins. At the heart of all of these efforts is a simple question: Exactly what mechanisms allow the continental lithosphere to be thinned to the point of rupture? Related questions are: (1) Do crustal and mantle faults play a major role in this thinning process? If so, what is their geometry and does their importance and geometry change with time? (2) Are there other mechanisms of lithospheric and crustal thinning that cannot be imaged on seismic sections? (3) How is deformation accommodated in space and time? (4) What role do inherited mechanical, thermal and/or chemical heterogeneities play in controlling strain distribution and localization? (5) When, how and to what degree does magma production affect the distribution and localization of extension? And (6) what is the stratigraphic record of continental extension and how does it document the extension of the crust and thinning of the lithospheric mantle? The aim of this Special Publication is to address many of these fundamental questions concerning the extreme extension and thinning of continental lithosphere.

The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?

We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such “off-axis” magmatism might be a common process in (ultra-) slow oceanic spreading systems, where “magmatic” and “tectonic” spreading varies in both space and time.

Observations from the Alpine Tethys and Iberia–Newfoundland margins pertinent to the interpretation of continental breakup

Abstract Although the Iberia–Newfoundland and Alpine Tethys margins are of different age and ultimately had a different fate, they share remarkable similarities. Both pairs of margins show a change from initially distributed and decoupled extension to later localized, coupled and asymmetric extension that results in thinning of the crust and exhumation of subcontinental mantle. The change in the mode of extension together with the localization of deformation reflects an evolution of the bulk rheology of the extending lithosphere. In this paper we summarize the pertinent geological observations for the Iberia–Newfoundland and Alpine Tethys margins. We describe the stratigraphic evolution, the fault geometry, basin architecture, and magmatic and metamophic evolution of the two pairs of margins from initial rifting to final continental breakup. This description forms a basis for understanding the evolution of the bulk rheology and how the various processes interact during progressive lithospheric extension. For the Iberia–Newfoundland and Alpine Tethys margins initial rifting appears to be controlled by inherited heterogeneities and mechanical localization processes, whereas final rifting and lithospheric rupture is controlled by serpentinization, magmatic and thermal weakening. At other margins, these modes may interact in a different way depending on the prerift conditions and the evolution of the rheology during rifting.

Correction to “Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin”

[1] In the paper ‘‘Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin’’ by Donna J. Shillington, W. Steven Holbrook, Harm J. A. Van Avendonk, Brian E. Tucholke, John R. Hopper, Keith E. Louden, Hans Christian Larsen, and Gregory T. Nunes (Journal of Geophys ica l Research , 111 , B09402, do i :10 .1029/ 2005JB003981, 2006), the last sentence of section 7 should be changed as follows: [2] Finally, the variable appearance and fault geometries within apparent oceanic basement on the Newfoundland margin suggest extension and an intermittent magma supply during the transition from nonvolcanic rifting to slow incipient oceanic accretion, similar to what is observed on adjacent SCREECH transects [Funck et al., 2003; Hopper et al., 2004; Lau et al., 2006a, 2006b]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B12403, doi:10.1029/2006JB004769, 2006 Click Here for Full Article

High degrees of melt extraction recorded by spinel harzburgite of the Newfoundland margin: The role of inheritance and consequences for the evolution of the southern North Atlantic

Serpentinized spinel peridotites of the Newfoundland margin drilled during ODP Leg 210 at Site 1277 have preserved, relic mineral compositions similar to the most depleted abyssal peridotites worldwide and different from those of the conjugate Iberian margin. The samples are derived from mass flows containing clasts of peridotite and gabbro and from in-situ basement, and are mostly mylonitic cpx-poor spinel harzburgites with Cr-rich spinels (Cr# 0.35–0.66). Melting of the Newfoundland mantle occurred in the spinel peridotite field and probably exceeded the cpx-out phase boundary for some samples. Using proposed spinel peridotite melting models and experimentally derived phase diagrams, the Newfoundland harzburgites can be modeled as a residue after extraction of 14 to 20–25% melting. Basalts that are interleaved with mass flow deposits on top of the peridotite basement resemble normal to transitional mid-ocean ridge basalt. This, together with the unusually high Cr# of some spinel harzburgites suggest that the formation of basalts and partial melting of the underlying peridotite are not cogenetic. Among mantle samples some of the Newfoundland harzburgites approach mineral compositions of the Bay of island ophiolite and ophiolites from Japan that represent peridotites formed in an arc-setting. Thus, the peridotites drilled at Site 1277 may represent inherited (Caledonian or older) subarc mantle that was exhumed close to the ocean floor during the rifting evolution of the Atlantic. Compared to the spinel harzburgites from Newfoundland, the peridotites from the conjugate Iberian margin are, on average, less depleted and provide evidence for local equilibration in the plagioclase stability field. This can either be explained by an inherited, primary, Ca-richer composition of the Iberia peridotite, or, alternatively, by local melt impregnation and stagnation during continental rifting, and thus refertilizing previously depleted (arc-related) peridotite.

Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin

Prestack depth migrations of seismic reflection data collected around the Ocean Drilling Program (ODP) Leg 210 transect on the Newfoundland nonvolcanic margin delineate three domains: (1) extended continental crust, (2) transitional basement, and (3) apparent slow spreading oceanic basement beyond anomaly M3 and indicate first‐order differences between this margin and its well‐studied conjugate, the Iberia margin. Extended continental crust thins abruptly with few observed faults, in stark contrast with the system of seaward dipping normal faults and detachments imaged within continental crust off Iberia. Transition zone basement typically appears featureless in seismic reflection profiles, but where its character can be discerned, it does not resemble most images of exhumed peridotite off Iberia. Seismic observations allow three explanations for transitional basement: (1) slow spreading oceanic basement produced by unstable early seafloor spreading, (2) exhumed, serpentinized mantle with different properties from that off Iberia, and (3) thinned continental crust, likely emplaced by one or more detachment or rolling‐hinge faults. Although we cannot definitively discriminate between these possibilities, seismic reflection profiles together with coincident wide‐angle seismic refraction data tentatively suggest that the majority of transitional basement is thinned continental crust emplaced during the late stages of rifting. Finally, seismic profiles image abundant faults and significant basement topography in apparent oceanic basement. These observations, together with magnetic anomaly interpretations and the recovery of mantle peridotites at ODP Site 1277, appear to be best explained by the interplay of extension and magmatism during the transition from nonvolcanic rifting to a slow spreading oceanic accretion system.

Age and geochemistry of the mafic sills, ODP site 1276, Newfoundland margin

Site 1276, Leg 210 of the Ocean Drilling Program, was located on the Newfoundland margin in a seismically-defined ∼ 128 Ma “transitional” crust just west of the presumed oceanic crust, and the M3 magnetic anomaly. The goal of drilling on this non-volcanic margin was to study the rifting, nature of basement, and post-rift sedimentation in the Newfoundland–Iberia rift. Drilling of this 1739 m hole was terminated 90–160 m above basement, in the lower of a doublet of alkaline diabase sills. We have carried out geochemical studies of the sill complex, in the hopes that they will provide proxy information regarding the nature of the underlying basement. Excellent 40Ar/ 39Ar plateau ages were obtained for the two sills: upper sill ∼ 105.3 Ma; lower sill ∼ 97.8 Ma. Thus the sills are substantially younger than the presumed age of the seafloor at site 1276 (∼ 128 Ma), and were intruded beneath substantial sediment overburden (250 m for the upper, older sill, and 575 m for the lower younger sill). While some of the geochemistry of the sills has been compromised by alteration, the “immobile” trace elements show these sills to be hawaiites, differentiated from an enriched alkaline or basanitic parentage. Sr, Nd and Pb isotopes are suggestive of an enriched hotspot/plume mantle source, with a possible “added” component of continental material. These sills unequivocally were not derived from typical MORB (asthenospheric) upper mantle.

Thermal alteration of terrestrial palynomorphs in mid-Cretaceous organic-rich mudstones intruded by an igneous sill (Newfoundland Margin, ODP Hole 1276A)

Most approaches used to reconstruct thermal alteration of sediments necessitate advanced, relatively expensive analytical techniques. We have evaluated the fidelity of a less costly, relatively simple approach of visually assessing sporomorph colours to determine thermal alteration. The sporomorph-based thermal alteration estimates were compared to vitrinite reflectance data from the same samples. As study material, we selected a succession of mid-Cretaceous (Albian) organic-rich clay- and siltstones intruded by a diabase sill that was recovered from Ocean Drilling Program (ODP) Hole 1276A, off Newfoundland. Six sporomorph groups (SG), each consisting of morphologically well-defined, easily identifiable constituents with long stratigraphic ranges, were individually evaluated for their thermal alteration signals. These groups are: (1) leiotrilete spores of the genera Biretisporites, Cyathidites, Deltoidospora, Dictyophyllidites, Gleicheniidites, and Leiotriletes (SG-1; subdivided into three subgroups SG-1a, SG-1b and SG-1c with sporoderm thicknesses 1.5 ?m, respectively); (2) trilete, rugulate spores of the genera Camerozonosporites and Lycopodiacidites (SG-2); (3) trilete, striate spores of the genera Appendicisporites, Cicatricosisporites and Plicatella (SG-3); and (4) the gymnosperm-pollen taxon Classopollis torosus (SG-4). Sporomorph colours were determined using Munsell colour standards under reproducible optical conditions. To minimize the potential influence of reworked specimens on the dataset, only the lightest 50% of all counted specimens per sporomorph group were evaluated for their thermal alteration signals. The thermal alteration estimates from all sporomorph groups yield an internally consistent picture that is compatible with vitrinite reflectance data from the same samples. They indicate that downhole thermal alteration does not increase until 20 m above the igneous sill. A steep rise occurs only at 4.23 m above the sill, and thermal alteration peaks in the sample closest (2.17 m) to the sill. However, the different sporomorph groups exhibit varying degrees of fidelity with respect to deciphering thermal alteration. Factors influencing the precision of the thermal alteration signal include sporoderm thickness, character of surface ornamentation, resistance to reworking, and abundance in the sample material. Highest correlations with vitrinite reflectance data are observed for the thermal alteration values from SG-1b (R=0.82), SG-3 (R=0.80) and SG-4 (R=0.80). Hence, these groups are best suited for a sporomorph-based approach to reconstructing the thermal history of sediments. The highest correlation coefficient with vitrinite reflectance data is registered for SG-1b, the subgroup with the least variability of sporoderm thickness and the highest abundance in the sample material. This indicates that the study of morphologically similar, highly abundant specimens with strongly constrained sporoderm thickness variations yields the best results for the reconstruction of thermal alteration.

Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3–4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to <1.3 km and exhibits an unusual, highly reflective layering. We propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep-marine sheet flows to form this layering. Subsequent seafloor spreading formed normal, ∼6-km-thick oceanic crust. This interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow-spreading system was established.

Petrographic evidence from the eastern Canadian margin of shelf-crossing glaciations

It is difficult to evaluate when and where glaciers extended across the outer continental shelf off eastern Canada. The outer shelf and upper slope are commonly hard bottomed or have a great thickness of post-glacial sediment, in either case making direct sampling and dating of ice-margin sediment impossible. The timing and extent of glacier ice is important for understanding continental slope sedimentation and ice sheet dynamics. We have therefore determined the lithologic components of the 1–4 mm granule fraction of over 100 well-dated samples from 40 cores from the more distal parts of the deep-water margin off Nova Scotia and Newfoundland. This fraction is demonstrably ice-rafted in these cores and geographic variation in component results from input of icebergs at different calving margins. The samples show that Heinrich layers have a distinctive granule fraction that shows some areal variation as a result of dilution by sources other than Hudson Strait. Heinrich event 2 (ca. 21 ka) is widespread as far southwest as the West Scotian Slope, but by the last glacial maximum (LGM), numerous discrete sources of ice-rafted granules are recognised, implying that calving ice margins were present in several places off Newfoundland and Nova Scotia, including off Banquereau. A distinctive purplish LGM gravelly mud is recognised on the eastern Newfoundland margin, derived from a calving margin on the NE Newfoundland Shelf. At the time of Heinrich event 1 (ca. 14 ka), there was a calving ice outlet somewhere on the Grand Banks and another in Halibut Channel. Retreat of ice from the Gulf of St. Lawrence can be tracked by changing granule petrology from the LGM to 13 ka. These data suggest that final ice retreat from many parts of the outer shelf was later than is shown in many recent ice-margin reconstructions.

The structure of the northeastern Gulf of St. Lawrence, Canada: new insight from geophysical data analysis

An improved compilation of magnetic and gravity data has been interpreted in conjunction with seismic reflection profiles to provide new information about the complex structure of the northeastern Gulf of St. Lawrence, Atlantic Canada. This region was affected by plate divergence and convergence events during the Grenville and Appalachian orogenies and the opening of the Iapetus Ocean. The Anticosti Basin, which developed as a foreland basin over the margin of Laurentia, is filled with a thick succession of Cambrian to Silurian sedimentary strata. Most of the interpreted magnetic and gravity anomalies have sources within the basement rocks, which is interpreted as Grenville crust beneath much of the study area. A V-shaped zone of lower amplitude gravity and magnetic anomalies in the center of the region is associated with a slight thickening of Cambrian to Middle Ordovician sedimentary rocks over a downthrown block of anorthositic Grenville crust, with a locally lower density and magnetization. Extensional faults bordering the zone presently display 130–250 m of downthrow at basement depths, increasing to the southeast, but show no disruption of strata younger than Middle Ordovician. A magnetic low 200 km to the northeast is of similar geophysical character and is associated with a similar geological structure. Numerous NE-trending normal faults associated with segmentation of the Grenville basement are manifested in the magnetic and seismic data. Related anomaly sources are also present within the overlying Ordovician calcareous and clastic rocks that were deposited during extension associated with the onset of the Taconian orogeny. Other anomalies are associated with faulting and folding of shallower strata, and seismic data indicate that some of the NE-trending faults were reactivated as thrusts towards the close of the Taconian orogeny in the Late Ordovician. The geophysical data show no evidence of significant deformation north of the western margin of Newfoundland that would be associated with later compressive events of the Acadian orogeny.

Crustal structure beneath Orphan Basin and implications for nonvolcanic continental rifting

A wide‐angle seismic profile was acquired across the northeast Newfoundland margin, seaward to Orphan Knoll. The profile is complemented by existing multichannel reflection data, two online deep wells, and other deep seismic reflection and refraction lines. Modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the ∼400‐km‐wide stretched continental crust on this margin. Prerift metasedimentary rocks have a topographic relief of up to 5 km, with a velocity of ∼5.2 km/s increasing to 5.9 km/s. Unaltered crystalline crust below the metasedimentary section has a velocity range of 6.1–6.5 km/s. Most of the lower crust landward of Orphan Knoll has a typical continental lower crustal velocity (6.8–7.0 km/s), and the lower crust shallows significantly beneath Orphan Knoll. Our model shows no evidence for the 5‐km‐thick, 7.35 km/s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating, implying nonvolcanic rifting for the NE Newfoundland margin. We suggest that continental stretching persisted from ∼180 Ma to Chron 34N time (∼101 Ma) when the final breakup between Canada and western Europe took place, leaving a 400‐km‐wide zone of thinned continental crust underneath the shelf and deep water area of the Orphan Basin. On the landward side of the basin, gravity modeling indicates a zone of very thin (6–8 km) crust, possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between Canada and Europe. A possible sub‐Moho reflector suggests generally uniform stretching of the crust and upper mantle, and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the Grand Banks.

Otway Continental Margin Transect: Crustal architecture from wide‐angle seismic profiling across Australia's southern margin

The Otway Basin in southeastern Australia formed on a triangular‐shaped area of extended continental lithosphere during two extensional episodes in Cretaceous to Miocene times. The extent of the offshore continental margin is highlighted by Seasat/Geosat satellite altimeter data. The crustal architecture and structural features across this southeast Australian margin have been interpreted from offshore‐onshore wide‐angle seismic profiling data along the Otway Continental Margin Transect extending from the onshore Lake Condah High, through the town of Portland, to the deep Southern Ocean. Along the Otway Continental Margin Transect, the onshore half‐graben geometry of Early Cretaceous deposition gives way offshore to a 5 km‐thick slope basin (P‐wave velocity 2.2–4.6 km/s) to at least 60 km from the shoreline. At 120 km from the nearest shore in a water depth of 4220 m, sonobuoy data indicate a 4–5 km sedimentary sequence overlying a 7 km thick basement above the Moho at 15 km depth. Major fault zones affect the thickness of basin sequences in the onshore area (Tartwaup Fault Zone and its southeast continuation) and at the seaward edge of the Mussel Platform (Mussel Fault). Upper crustal basement is interpreted to be attenuated and thinned Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. Basement rocks comprise a 3 km section with velocity 5.5–5.7 km/s overlying a deeper basement unit with velocity 6.15–6.35 km/s. The Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the transect (water depth 5200 m). The continent‐ocean boundary is interpreted to be at a prominent topographic inflection point 170 km from shore at the bottom of the continental slope in 4800 m of water. P‐wave velocities in the lower crust are 6.4–6.8 km/s, overlying a thin transition zone to an upper mantle velocity of 8.05 km/s beneath the Moho. Outstandingly clear Moho reflections seen in deep‐marine profiling data at about 10.3 s two‐way time under the slope basin and continent‐ocean boundary place further strong controls on crustal thickness. There is no evidence of massive high velocity (>7 km/s) intrusives/underplate material in the lower crust nor any synrift or early post‐rift subaerial volcanics, indicating that the Otway continental margin can be considered a non‐volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins e.g. the Nova Scotia ‐ Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula. Using this analogue, the prominent gravity feature trending northwest‐southeast at the continent‐ocean boundary may indicate the presence of highly serpentinised mantle material beneath a thin crust, but this has yet to be tested by detailed work.

Lithospheric wide‐angle seismic profiles using stacked airgun shots

The signal‐to‐noise ratio of marine wide‐angle seismic profiles can be significantly enhanced by stacking multiple shots. Signals detected from airgun shots from a seismic ship repeated many times within a small source area (called “stack shots”) can be stacked in a manner somewhat similar to common‐mid‐point processing of reflection seismic data. We collected two such “stack shot” profiles across the eastern margin of Newfoundland. At each shooting site, 36 closely spaced airgun shots were fired consecutively, and recorded along a profile made of about 400 land receivers at offsets of 100 to 455 km. While the data can be stacked in several different ways we show that a two‐step technique or “two‐pass stack” is the most effective. The traces of each receiver gather are first stacked at an aperture of 0.5 km along a slope of 8 km/s (stacking with linear moveout). The stacked traces are then reordered by increasing offset and stacked a second time along the same slope and with an aperture of 0.5 km or less. This technique is superior to a direct stack in which all the traces would be stacked in one pass because it allows improvement in the data quality by the various methods designed for each phase of the two‐pass stack. Our results show that the “stack shot” technique coupled with the “two‐pass stack” is a viable alternative, with less environmental impact, to using large, single explosions at sea.

Paleo heat flow of eastern Canada's passive margins

Tectonic subsidence rates, derived from several wells in the Labrador Sea and offshore Nova Scotia and Newfoundland, were used to estimate the paleo heat flow along the margins of eastern Canada. If tectonic subsidence is caused by the cooling of the lithosphere only, the transient component of the heat flow (i.e., the heat flow in excess of background heat flow) can be estimated directly from subsidence data. For the Nova Scotia and the Labrador Sea margins, the analysis shows that the transient component of the heat flow decreased markedly from 28–56 mW · m −2, immediately after rifting, to a present value between 7 and 14 mW · m −2 depending on boundary conditions. The analysis shows a distinctive difference between the evolution of the Labrador Sea and that of the northeastern Newfoundland margin where it is estimated that the transient component of the heat flow exceeded 100 mW · m −2 after rifting and dropped rapidly to very low values. The study suggests that the subsidence of the Newfoundland margin was not caused by cooling only but required an additional mechanism such as lower-crustal phase transformations or continuing extension after continental breakup with ductile deformation in the lower crust and upper mantle. The estimated transient heat flow was used to calculate the average present surface heat flow in eastern Canada's margins, which is the sum of heat flow from the mantle, crustal heat production, and the transient cooling of the lithosphere. The calculated heat flow compares well with heat flow density data obtained from bottom hole temperature measurements.

Results from a detailed aeromagnetic survey across the northeast Newfoundland margin, Part II: Early opening of the North Atlantic between the British Isles and Newfoundland

Poles of rotation for the North Atlantic have been derived from the results of a new aeromagnetic survey northeast of Newfoundland. Reconstruction of the North Atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the Atlantic from Flemish Cap and Goban Spur to the Azores-Gibraltar Fracture Zone. A group of similar anomalies has also been identified in the Bay of Biscay. North of Goban Spur and Flemish Cap, these anomalies follow the ocean-continent boundary. Poles of rotation derived for this anomaly show that it forms an isochron (100–110 m.y.) during the long Cretaceous normal polarity interval. The cause of this anomaly is not definite, but it may represent an increase in the magnetization of the crust during a limited time within the Cretaceous Magnetic Quiet Zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by Raymond and LaBrecque. The North Atlantic has also been reconstructed at the time of the initial opening in the region between Flemish Cap and the Charlie-Gibbs Fracture Zone, using inferred ocean-continent boundaries on the west and east sides: it has been shown that the entire region could not have saparated at one time, but that spreading between the British Isles and Newfoundland had to progress from south to north. Consequently, when active sea-floor spreading was taking place between Goban Spur and Flemish Cap (about 110 m.y.) the region to the north was still being stretched. The calculated amount of stretching as derived from the reconstructions (about 25%) agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region, and with the results of deep seismic studies. Active spreading in the north started about 100 m.y. ago.

Results from a detailed aeromagnetic survey across the northeast Newfoundland margin, Part I: Spreading anomalies and relationship between magnetic anomalies and the ocean-continent boundary

A detailed aeromagnetic survey carried out across the northeast Newfoundland margin clearly shows the presence of sea floor spreading anomalies 25 to 34. Correlation of these anomalies with synthetic profiles shows an increase in the rate of spreading soon after anomaly 27 time. Three fracture zones can be identified by dislocations in the magnetic anomalies; their positions are confirmed on the depth to basement map of this region. An eastward extension of the southernmost fracture zone at latitude 49 N matches well with the Faraday Fracture Zone across the Mid Atlantic Ridge, and with a basement ridge known as Pastouret Ridge mapped off Goban Spur. By combining the present survey data with the previously collected shipborne measurements, we have also traced the westward continuation of the Charlie-Gibbs Fracture Zone under the Newfoundland shelf. A large amplitude magnetic anomaly lies along the margin and separates two zones with different magnetic characteristics: long wavelength small amplitude anomalies on the landward side, and quasi lineated anomalies on the seaward side. Seismic data compilations show that this large anomaly coincides with the ocean-continent boundary at most places north of Flemish Cap. Modelling of the magnetic anomalies indicate that the large amplitude anomaly is caused by the juxtaposition of highly magnetized oceanic crust against weakly magnetized continental crust; this situation is similar to that observed across the Goban Spur margin, which is a conjugate of the Flemish Cap margin. The presence of highly magnetized oceanic crust landward of anomaly 34 and within the Cretaceous Magnetic Quiet Zone is attested to by the presence of similar large amplitude anomalies south of the Flemish Cap and Goban Spur regions, but these do not mark the ocean-continent transition.