Background and general hypothesis:

The tectonic and stratigraphic evolution of the Grand Banks resulted from a number of rifting events beginning in the late Triassic and culminating in the Cenomanian. Limited marine magnetic anomaly data and DSDP/ODP drilling results indicate that seafloor spreading between the central Grand Banks and Iberia began in early Aptian time, while farther north, between Flemish Cap and Goban Spur, spreading began by at least late Albian time. Previously identified Aptian and Cenomanian unconformities of the Jeanne d'Arc basin are of regional extent and, in places, are associated with pronounced erosion. These unconformities are generally interpretated to be break-up unconformities because of their close temporal link with the rift/drift transition.

Previous interpretations of seismic reflection and drilling data from the Jeanne d'Arc basin suggested several phases of rifting. The first phase of rifting commenced in the late Triassic with minimal block motion. The basin subsided uniformly through much of the early and middle Jurassic accumulating thick sequences of carbonates and calcareous mudstones. Block motion in the Callovian marked the onset of the second phase of rifting and thick fluvial and paralic sequences infilled the developing basins. During the Barremian, the entire sediment carapace above the Argo salt was displaced to the northeast along a "supra crustal" detachment in response to differential subsidence in the Jeanne d'Arc Basin. Well data indicate that subsidence increased from south to north tilting the entire basin to the northeast. The Aptian and Cenomanian unconformities of the Jeanne d'Arc basin and the adjacent parts of offshore eastern Canada are of regional extent and in places are associated with pronounced erosion. Sea-floor magnetic anomaly data suggest that these unconformities correlate with, or are only slightly younger than, the onset of sea-floor spreading adjacent to different segments of the Grand Banks. Given the close temporal relation between the ages of the Aptian and Cenomanian unconformities in the Grand Banks and the timing of sea-floor spreading, they have been interpreted to be the so called "break-up unconformities". Our overall objective was to test the hypothesis that at least some regional unconformities, particularly break-up unconformities, may be related to variations of in-plane stress within the lithosphere. Our proposed research was to determine if the Aptian and Cenomanian break-up unconformities in the Jeanne d'Arc basin were generated by in-plane force variations caused by the progressive cessation of extension around the Grand Banks or by eustatic sea-level fluctuations.

Using PetroCanada seismic reflection and exploratory well data from the Jeanne d'Arc basin, we have mapped the stratigraphic patterns, the distribution of these unconformities, and established a chrono- stratigraphic framework for the depositional history of the basin.

Key results are:

(1) The late Barremian unconformity is regionally developed across the Grand Banks. The divergence of seismic reflectors above the unconformity attests to differential subsidence. The previously interpreted Aptian break-up unconformity is therefore a rift-onset unconformity documenting this late Barremian phase of extension. In our interpretation, rifting in the Jeanne d'Arc basin continues to at least Albian time. The break-up unconformity to this last phase of rifting is dated as Cenomanian. The post Cenomanian sediment would then represent the thermal phase of subsidence following the last phase of rifting on the Grand Banks. Sediment overlying the late Barremian unconformity does not onlap the Hibernia structural high implying that the uplift of the Hibernia structure must have occurred after deposition of these sediments.

(2) The cessation of rifting was marked by the generation of a late Albian unconformity. Erosional truncation associated with this surface is best developed along the western potion of the transfer zone (Nautilus transfer zone) near the Hibernia and Flying Foam structures. Unconformity generation is the consequence of the reactivation and inversion of crustal blocks across the transfer zones and along pre-existing faults. Orientation of the late Albian in-plane compression was north-northeast, parallel to the strike of the Jeanne d'Arc basin. Thus, the deformation associated with the rift/drift transition is dominated by "inversion tectonics" and it is unlikely that we will be able to recognize any elastic deformation due to in-plane force variations.

(3) The Murre and Mercury border faults form the western boundary of the Jeanne d'Arc basin. They are curvilinear, strike north-northeast, and are segmented by transfer zones. Each segment of the border fault is curved and in map view, each border fault is "spoon-shaped". The largest offset of the Murre border fault occurs across the Nautilus Transfer zone (18 km) and results in a structural high that is best developed in the west and progressively diminishes eastward into the basin. The amount of Cenomanian erosion also varies along this transfer zone. An east-west profile across the border fault would reveal an eastward-dipping listric fault. In contrast, a north-south cross-section across the western portion of the transfer zone shows a southward dipping listric fault south of the zone, and a northward dipping listric fault north of the zone. In map-view, each border fault segment is "spoon-shaped".

(4) The Hibernia and Flying Foam structures developed above these "spoon-shaped" faults. The uplift, rotation, and truncation of the Hibernia and Flying Foam structures suggest that they were formed by minor in- plane compression generated across the Nautilus Transfer zone during the Cenomanian. The timing of compression correlates with the cessation of extension on the Grand Banks. Crustal blocks were reactivated along earlier normal faults thereby inverting the overlying sediments. The degree of uplift and truncation diminishes eastward and so there are no equivalent sedimentary structures across the transfer zone in the eastern portion of the basin.

(5) Basin inversion and deformation were focussed along pre-existing structure, in particular the accommodation zone between curvilinear segments of the Murre border fault. Subsequent erosional truncation produced the Cenomanian break-up unconformity.

 

Conclusions:

Lithospheric in-plane force variations have been proposed as a tectonic mechanism to generate unconformities. If this hypothesis is correct, tectonic events such as lithospheric compression and extension will result in relative sea-level changes. Testing this hypothesis has been difficult due to problems in defining both the timing and magnitude of in-plane force variations necessary to generate relative sea-level changes. However, lithospheric compression induced by the cessation of extension on the Grand Banks provides an ideal opportunity to test this hypothesis by determining the timing and magnitude of in-plane force variations and the resulting lithospheric deformation. The break-up unconformity, such as the Cenomanian unconformity on the Grand Banks, separates syn- from post-rift sediments. Seismic reflection and exploratory well data from the Jeanne d'Arc Basin reveal that the erosional truncation associated with this unconformity is best developed along accommodation zones. Topographic relief across these accommodation zones was produced by Cenomanian reactivation and inversion of crustal blocks along pre-existing faults. Yield-stress envelopes describing the rheological properties of extending lithosphere imply in- plane forces of 0.5-1x10¹³ N/m. Thus in the Jeanne d'Arc Basin, the only recognizable response of the lithosphere to the change of in-plane force during the rift/drift transition is a brittle non-elastic deformation associated with the reactivation of crustal blocks.

We can contrast this response by examining the deformation of the Central Indian Ocean due to the application of in-plane compression engendered by the collision of India with Eurasia. Modeling by Karner and Weissel (1990) demonstrated that the compression-related intraplate deformation zone of the Central Indian Ocean nucleated about a pre-existing deformation within the lithosphere, namely, the Afanazy-Nikitin Seamounts. Information concerning the timing and magnitude of in-plane force variations come from recent ODP LEG 116 drilling. Interpretation of the results from Leg 116 drilling suggests that intra-plate deformation around the Afanazy-Nikitin seamounts in the Central Indian Ocean began in the late Miocene and is recorded by long-wavelength crustal folds (200- 250 km) with an amplitude of 1-2 km, and shorter-wavelength (5-20 km) reverse faulting of the crust and overlying sedimentary section. This reverse faulting represents inversion of a pre-existing fabric imparted during the formation of oceanic crust at the ridge crest. Gravity modeling suggests that the level of in-plane force is approximately 10¹³ N/m. Thus, in contrast to the Jeanne d'Arc example where the dominant response to in-plane compression is the inversion of crustal blocks (and inelastic process), the features generated in the Central Indian Ocean are a consequence of both elastic and inelastic deformations.

Based on these results, we conclude that both elastic and inelastic deformations are produced by changes of in-plane force variations and therefore are capable of causing relative sea-level changes. However, we do not expect a simple relationship between basin shape, the distribution and magnitude of any in-plane engendered deformation, and the distribution of unconformities. Although it is tempting to correlate in-plane force variations and the 3rd-order sea level cycles for entire plate systems, we believe that in-plane force variations are not the major contributors to the observed sea level cyclicity.

The onset of seafloor spreading between the central Grand Banks and Iberia is uncertain because of limited marine magnetic and drilling data (ODP & DSDP), and the existence of the Cretaceous magnetic quiet zone along the margin. However, recent studies indicate that magnetic anomaly M0 (118 Ma) is not well resolved north of the Newfoundland Seamounts within the Newfoundland basin and is not present north of the Figueiro fracture zone along the conjugate Iberian margin. This suggests that seafloor spreading between the northern portion of the Newfoundland basin and the northern Iberian margin began after the early Aptian. Given that the cessation of rifting marks the onset of seafloor spreading, our seismic sequence analysis indicates that the onset of seafloor spreading in the northern Newfoundland basin, north of the Newfoundland Seamounts, began after late Aptian time.

The collapsed hangingwall block of the Jeanne d'Arc basin is a classic roll- over anticline.

The following seismic interpretation is an excellent example of both extensional and inversion structures; a collapsed hangingwall, divergent seismic reflectors, a series of onlap surfaces, and the Hibernia inversion structure . . . . . .