Sonya A. Dehler

Crustal structure of the northern Nova Scotia rifted continental margin (eastern Canada)

[1]xa0The Nova Scotia continental margin off eastern Canada marks a transition from a volcanic to a nonvolcanic style of rifting. The northern (nonvolcanic) segment of the margin was studied by a 490-km-long refraction seismic line with dense air gun shots, coincident with previous deep reflection profiles. A P wave velocity model was developed from forward and inverse modeling of the wide-angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles. The continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5.7–6.9 km/s. Crustal thinning down to 3 km occurs in a 180-km-wide zone and the sediment cover in this area is up to 15 km thick. Farther seaward, a 150-km-wide transition zone is observed with a 5-km-thick lower layer (7.2–7.6 km/s) interpreted as partially serpentinized mantle. At the landward end, this layer is overlain by highly altered continental crust (5.4 km/s) extending up to the seaward limit of the Jurassic salt province. Farther seaward, the upper layer is interpreted as exhumed and highly serpentinized mantle (5.1 km/s) separated from the lower layer by subhorizontal reflectivity, which probably represents a serpentinization front. Oceanic crustal thickness is 4 km with layer 2 velocities of 4.6–5.0 km/s. Layer 3 velocities of 6.4–6.55 km/s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile. This reduced magma production might be related to the proximity of the Newfoundland transform margin.

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.

The volcanic margins of the northern Labrador Sea: Insights to the rifting process

[1]xa0A new interpretation of seismic reflection data on the continental margin of northern Labrador shows basement structures similar to those observed on other volcanic rifted margins. Seaward dipping reflections, inner flows, volcanic plateaus, and lava deltas are observed on these data. Magnetic chron 27n (∼61 Ma) is coincident with the volcanic plateaus, connecting these features to the Paleocene volcanism farther north in Davis Strait. Therefore, we are able to extend the region of volcanism about 500 km south along the margin. Similar structures are also observed on the conjugate west Greenland margin. Below the volcanic plateaus, gravity and wide-angle seismic data show thick igneous crust, which is fairly symmetrically distributed across the conjugate margins. However, the geometry of the thinned continental crust is not symmetrical: a narrower zone of thinning is observed off northern Labrador. The thick igneous crust lies seaward of thinned continental crust, which exhibits little or no underplating by the magmatic event. This observation is compatible with recent models for the formation of nonvolcanic margins, followed by a magmatic event late in the rift evolution. In Mesozoic time rifting of the cold, thick cratonic lithosphere occurred between Greenland and North America with minimal volcanic activity. Then in Late Cretaceous time, the zone of rifting narrowed and localized seaward of the shelf, creating a relatively narrow zone of lower lithospheric thinning and mantle upwelling into which the Paleocene magmas were eventually channeled. This scenario is supported by studies of Mesozoic igneous rocks on land in coastal west Greenland and Labrador.