C. E. Keen

Deep seismic reflection profile across the northern Appalachians

A marine seismic reflection line across the northeast extremity of the Canadian Appalachians in Newfoundland indicates a collisional suture(?) in the lower crust beneath the central ophiolitic Dunnage terrane. The thrust and fold belt (miogeocline) above the Grenville basement, and the Dunnage and Gander tectonostratigraphic terranes all appear to be allochthonous with respect to lower crustal basement. The Gander-Avalon terrane boundary to the east is a near-vertical feature that penetrates the crust. The data also suggest that the ancient passive margin of North America extends eastward under the Dunnage terrane for about 70 km. The Newfoundland deep seismic experiment indicates major tectonic differences compared to results for the southern Appalachians.

The transition from a volcanic to a nonvolcanic rifted margin off eastern Canada

Three multichannel seismic reflection profiles were collected on the rifted continental margin southeast of Nova Scotia, eastern Canada. The profiles cross the East Coast Magnetic Anomaly (ECMA), which parallels much of the margin of eastern North America south of the Grand Banks and which is usually associated with the transition from continental to oceanic crust. Studies to the south of the work reported here suggest that the ECMA may be related to the emplacement of large thicknesses of late rift stage or early drift stage igneous material which is characterized by seaward dipping reflections in basement and a high-velocity lower crustal layer. The seismic data show that seaward dipping reflections (SDR) continue northward into the study area and support the correlation between the SDR unit and the presence of a well-developed ECMA. Magnetic modeling confirms this association, although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture. Just north of the study area the ECMA diminishes and is no longer well developed. The SDR unit also terminates and it is not observed over most of the Nova Scotian margin. If our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north. The association of the transition with significant changes in the prerift fabric of the adjacent continental crust, in the trend of synrift extensional structures, and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced. We suggest that these clues are consistent with small-scale convection as a mechanism for delivering large melt volumes to crustal depths during rifting.

A Comparison of Foreland and Rift Margin Sedimentary Basins

Foreland and rift margin basins are compared on the basis of (1) their tectonic setting, (2) reasons for subsidence, and (3) their large-scale geophysical and geological characteristics. The thermal and mechanical properties of the underlying lithosphere are shown to be fundamental to the form of tectonic subsidence. The lithosphere beneath foreland basins is flexurally downwarped by the loading of the adjacent fold-thrust belt, whereas tectonic subsidence at rifted margins is caused by mass replacement at depth, during lithospheric extension on rifting, and subsequent thermal contraction as the lithosphere cools. The effects of rheology, thermal maturity, and lateral changes in properties of the lithosphere are outlined for foreland basins, as is the topographic effect of possible phase changes beneath the fold-thrust belt. The thermal and rheological consequences of lithospheric extension at rift margins makes flexural subsidence relatively less important than in foreland basins. Flexure may, however, be partly responsible for uplift landward of the hinge line that is associated with rifting. Other mechanisms that could cause such uplift include depth-dependent extension and thermal expansion due to the lateral diffusion of heat. The models describing the evolution of these basins are shown to predict characteristics that are in accord with observations. The superposition of foreland and rift margin basins as a result of ocean closure can lead to an overall basin stratigraphy that is complex. Such phases of basin subsidence must be separated according to the tectonic environment in which they formed in any analysis of the cause and consequences of basin evolution.

Conjugate margins of Canada and Europe: Results from deep reflection profiling

Deep seismic reflection profiles have been collected across the conjugate margins of the North Atlantic Ocean. The eastern North Atlantic margin is traversed in the Goban Spur region, and the western North Atlantic margin is crossed in the vicinity of Flemish Cap in the Grand Banks region. These seismic profiles allow us to examine the deep structures and mode of extension in crust that was once contiguous. The Flemish Cap and Goban Spur margins have different structural styles: thick and relatively unfaulted crust is present on the Flemish Cap margin west of the continent-ocean boundary, whereas the Goban Spur margin exhibits a zone of extensional faulting and thinned continental crust. The restored rift zone displays overall symmetry in which a bridge of thin crust (< 15 km thick), about 60 km wide, joins the two crustal blocks of normal continental thickness (∼28 km). The configuration of the Moho, the geometry and distribution of the lower crustal reflections, and the overall symmetry of the rift zone favor pure-shear stretching in the lower lithosphere, although either pure or simple shear could accommodate crustal extension.

The canadian atlantic margin: A passive continental margin encompassing an active past

Haworth, R.T. and Keen, C.E., 1979. The Canadian Atlantic margin: a passive continental margin encompassing an active past. In: C.E. Keen (Editor), Crustal Properties across Passive Margins. Tectonophysics, 59: 83–126. The continental margins of Atlantic Canada described in this paper show the effects of plate tectonic motions since Precambrian time and thus represent an ideal natural laboratory for geophysical studies and comparisons of ancient and modern margins. The Grenville Province shows vestiges of Helikian sedimentation on a pre-existing continental block beneath which there may have been southeastward late-Helikian subduction resulting in collision between the Grenville block and the continental block comprised of the older shield provinces to the north. The Grenville block was subsequently split in Hadrynian time along an irregular line so that the southeastern edge of the Grenville exhibited a series of promontories and re-entrants similar to those seen at the present Atlantic continental margin of North America. That margin, which had a passive margin history perhaps comparable with that of the present Atlantic margin, was separated by the Iapetus ocean from the Avalon zone whose Precambrian volcanism has been attributed both to that associated with an island arc and with intra-cratonic rifting. However, the Iapetus ocean appears to have been subducted in early Paleozoic time with a southeastward dip beneath the Avalon zone, leaving exposures of oceanic rocks in place as in Notre Dame Bay, or transported onto Grenville basement as at Bay of Islands. Plate motions proposed for Devonian and Carboniferous time are numerous, but resulted in the welding of the Meguma block to the Avalon zone of New Brunswick and northern Nova Scotia, extensive faulting within Atlantic Canada which can be correlated with contemporaneous European faulting, and extensive terrestrial sedimentation within the fault zones. Graben formation, continental sedimentation and basaltic intrusion in the Triassic represent the tensional prelude to the Jurassic opening of the present Atlantic Ocean. This Jurassic opening produced a rifted margin adjacent to Nova Scotia and a transform margin along the southern Grand Banks. The width of the ocean—continent transition across the transform margin (approx. 50 km) is narrower than for the rifted margin (approx. 100 km). The eastern part of the transform margin is associated with a complex Cretaceous (?) volcanic province of seamounts and basement ridges showing evidence of subsidence. The western portion of the transform margin is non-volcanic, adjacent to which lies the 350 km wide Quiet Magnetic Zone floored by oceanic crust. Development of the margin east of Newfoundland was more complicated with continental fragments separated from the shelf by deep water basins underlain by foundered and atypically thin continental crust. Although thin, the crust appears unmodified, the similarities between the crustal sections of the narrow Flemish Pass and the wide Orphan Basin suggesting that the thinning is not simply due to stretching. The Newfoundland Basin shows evidence for two-stage rifting between the Grand Banks and Iberia with both lateral separation and rotation of Spain, leaving a wide zone of transitional crust in the south. The overall pattern of variations in crustal section for the margin east of Newfoundland is comparable with that of the British margin against which it is located on paleogeographical reconstructions. The major sedimentary unconformities on the shelves (such as the Early Cretaceous unconformity on the Grand Banks) reflect uplift accompanying rifting. Tracing of the sedimentary horizons across the shelf edge is complicated by paleocontinental slopes, which separate miogeocline and eugeocline depositional environments. The subsidence of the rifted margins is primarily due to cooling of the lithosphere and to sediment loading. The subsidence due to cooling has been shown to vary linearly with (time)1/2, similar to the depth—age behaviour of oceanic crust. The consequent thermal history of the sediments is favourable for hydrocarbon generation where other factors do not preclude it.

A deep seismic reflection profile across the extinct Mid-Labrador Sea spreading center

In 1990 a deep multichannel seismic reflection line was shot along a flow line across the Labrador Sea. The results from the central portion of this line between magnetic anomalies 25 and across the extinct central ridge are described here. Spreading rates in this part of the Labrador Sea are very low, from 10 mm/yr to less than 3 mm/yr, so that the line provides a unique opportunity to examine the relationship between very slow spreading and crustal structure. A clear division is observed between two types of crust. The oldest crust, between dirons 21 and 25, which formed at a mean half spreading rate of 10 mm/yr, exhibits smoothly undulating basement with only minor normal faulting. This region shares many of the reflection characteristics of North Atlantic crust formed at moderate to low spreading rates. In contrast, the younger central region, between chrons 21 and 13, which formed at a mean half spreading rate of 3 mm/yr, displays evidence of intense normal faulting of the crust, giving a total extension of about 70%. Inward facing normal faults on both sides of the extinct ridge, with large offsets, many of which extend to lower crust or Moho depths, dominate the seismic section. The axial region is characterized by a deep, fault-bounded, median valley. These results suggest that mechanical extension plays a more important role in seafloor spreading at low spreading rates than previously documented. Integration of the reflection data with previous refraction measurements and with gravity modeling of the region shows variations in crustal thickness which can be correlated with spreading rates. The region formed at a mean spreading rate of 10 mm/yr, where about 15% extension is observed, exhibits slightly thinner than normal crust (4.8 km or less versus a normal thickness of about 7 km). At a lower spreading rate of 3 mm/yr across the axial region where extension is about 70%, an average crustal thickness of 3 km is obtained. Thus lower spreading rates are associated with regions of thinner crust and greater amounts of extension. While many studies suggest that thin crust at slow spreading rates may result from a reduced magma supply, this study suggests that extension is at least equally important and may be responsible for most of the variations in crustal thickness. The increased cooling of young oceanic lithosphere formed at these very low spreading rates (approximately 3 mm/yr) may have amplified brittle failure in response to plate separation. However, the timing of extension is still uncertain and some of it may be related to postextinction tectonics and not to the spreading process. The role of extension as a control on crustal thickness needs to be considered further in studies of crustal generation and magmatism at slow spreading ridges. Extension will decrease the importance of magmatism in generating thin oceanic crust and will favor models of magmatic processes which produce thicker crust.

Stretching and subsidence: Rifting of conjugate margins in the North Atlantic Region

Subsidence on rifted conjugate continental margins around the North Atlantic is analyzed to derive the amount and areal distribution of stretching in the crust and in the lower lithosphere during continental rifting. Study areas are the Grand Banks and Orphan Basin regions of the eastern Canadian continental margin and the Goban Spur and Galicia Bank regions off western Europe. In all areas, maps of synrift and postrift sediment thickness and bathymetry were used to derive maps of post- and synrift subsidence. A two-layer lithospheric stretching model with independent amounts of stretching in the crust and in the lower lithosphere was assumed to be applicable, with the rifting history approximated by several instantaneous episodes of extension. This model was used to derive estimates of stretching at all points on a 0.05° geographical grid, where subsidence values were available within the study regions. The models are constrained with seismic measurements of crustal thickness. The results imply that pure shear stretching predominates at a lithospheric scale, while simple shear is more localized laterally and confined to the crust. In places there is significant decoupling between crustal and mantle stretching. Near the continent-ocean boundary, final continental breakup may be localized on one side of the rift between conjugate margin pairs, rather than symmetrically located. Total extension of the margins is compatible with that estimated from normal fault geometries and indicates that the continent-ocean boundary has been extended up to 350 km seaward of its original position, which should be considered in plate kinematic reconstructions.

Extensional styles and gravity anomalies at rifted continental margins: Some North Atlantic examples

Regional isostatic adjustment of the buoyancy forces created by lithospheric stretching during rifting is used to predict the crustal structure and gravity anomalies across rifted continental margins. Following earlier studies, we assume that stretching and necking of the lithosphere occurs around a “depth of necking,” which is the level of no vertical motion in the absence of gravitational forces. Differences in the depth of necking, coupled with lateral variation in flexural rigidity, can account for many of the variations in tectonic style observed across rifted continental margins and associated rifted basins. We investigate here seven transects crossing the rifted margin around the North Atlantic which display considerable variations in subsidence, crustal thickness variations, and gravity signatures. These are located where high-quality seismic data are available as a constraint. Two conjugate margin segments are included to test for asymmetry in depth of necking which might be evidence of a simple shear mode of extension. Results suggest that both shallow (3 to 10 km) and deep (20 to 25 km) depths of necking occur. The depth of necking appears to be related to the intrinsic strength maximum within the lithosphere, rather than to the depth of preexisting structure. Shallower depths of necking may result from heating of the lithosphere during extension which decreases the depth of maximum strength. Deeper depths of necking may occur when the rates of extension are low and significant heating of the lithosphere does not occur. The depth of necking on at least one margin transect gives results very similar to a locally (Airy) compensated model, even though the lithosphere exhibits finite strength. Both conjugate margin segments display shallow depths of necking and favor a pure shear rather than a simple shear mode of extension.

Formation and evolution of the Nova Scotian rifted margin: Evidence from deep seismic reflection data

A deep marine seismic reflection profile was obtained across the Mesozoic rifted continental margin off Nova Scotia, eastern Canada. This profile crosses the Seotian Basin, one of the deepest basins on the margin of eastern North America, and it complements other deep crustal seismic data on this margin. The seismic data have been interpreted in conjunction with gravity anomaly and subsidence data. They show significant thinning of the continental crust over a zone about 200 km wide. The mode of extensional deformation is probably a combination of pure and simple shear; there is evidence for simple shear in the crust. The continent-ocean boundary lies near the seaward edge of synrift salt below the continental rise. A 100-km-wide zone of very thin (approximately 9 km or less) continental or transitional crust extends seaward from the outer shelf to this boundary. Reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction, perhaps modified by extensional faulting. This margin is nonvolcanic, and the transition to the volcanic margin off the eastern United States occurs about 500 km southwest of the seismic line. The width of the zone of crustal extension is much greater on this nonvolcanic margin segment than it is on the volcanic margin to the south. It seems likely that the prerift fabric of the continental lithosphere controls this width. A narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism. However, significantly narrower zones of crustal extension occur on other nonvolcanic margins, so factors in addition to rift width, such as the rate of rifting, may also be important.

Crustal anatomy of a transform continental margin

Abstract This paper describes new deep Seismic reflection results across the major transform margin southwest of the Grand Banks, off eastern Canada. An interpretation of these results is presented, supplemented with previous Seismic refraction, gravity and magnetic measurements. The results show a fairly sharp transition between oceanic and continental regions at the transform margin. A narrow zone, about 40 km wide, separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust. Faults in this region extend throughout the crust and there is some evidence that a major strike-slip fault may extend some tens of kilometres into the upper mantle. Magmatism may have further modified the crust in this shear zone, which is bounded on the oceanic side by a small seamount. The continental crust is thinned by a factor of 3 or more in this zone, which may be due to erosion or to flow in the lower crust during shearing. The oceanic crust does not appear to thin significantly as the transform margin is approached. Crustal-scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two Appalachian terranes: the Meguma and Avalon terranes. This supports earlier interpretations of a large magnetic lineament in the region as marking this boundary. These crustal Seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins. The picture they provide is different in many respects from that across rifted margins and similar in others. In some cases it might be difficult to distinguish between these two classes of Atlantic-type margins on the basis of crustal structure.