ALBERT

Description: The NewfoundlandIberia rift is considered to be a type example of a non-volcanic rift. Key features of the conjugate margins are transition zones (TZs) that lie between clearly continental crust and presumed normal (Penrose-type) oceanic crust that appears up to 150180 km farther seaward. Basement ridges drilled in the Iberia TZ consist of exhumed, serpentinized peridotite of continental affinity, consistent with seismic refraction studies. Although the boundaries between continental crust and the TZs can be defined with relative confidence, there are major questions about the position and nature of the change from rifting to normal sea-floor spreading at the seaward edges of the TZs. Notably, drilling of presumed oceanic crust in the young M-series anomalies (〈M5) has recovered serpentinized peridotite, and this basement experienced major extension up to approximately 15 million years after it was emplaced. In addition, existing interpretations place the breakup unconformity' (normally associated with the separation of continental crust and simultaneous formation of oceanic crust) near the AptianAlbian boundary, which is also some 15 million years younger than the oldest proposed oceanic crust (anomaly M5M3) in the rift. To investigate and potentially resolve these conflicts, we analysed the tectonic history and deep (pre-Cenomanian) stratigraphy of the rift using seismic reflection profiles and drilling results. Rifting occurred in two main phases (Late Triassicearliest Jurassic and Late JurassicEarly Cretaceous). The first phase formed continental rift basins without significant thinning of continental crust. The second phase led to continental breakup, with extension concentrated in three episodes that culminated near the end of Berriasian, Hauterivian and Aptian time. The first two episodes appear to correlate with separation of continental crust in the southern and northern parts of the rift, respectively, suggesting that the rift opened from south to north in a two-step process. The third episode persisted through Barremian and Aptian time. We suggest that during this period there was continued exhumation of subcontinental mantle lithosphere at the plate boundary, and that elevated in-plane tensile stress throughout the rift caused intraplate extension, primarily within the exhumed mantle. This rifting may have been interrupted for a time during the Barremian when melt was introduced from the southern edge of the rift by plume magmatism that formed the Southeast Newfoundland Ridge and J Anomaly Ridge, and the conjugate MadeiraTore Rise. We propose that the rising asthenosphere breached the subcontinental mantle lithosphere in latest Aptianearliest Albian time, initiating sea-floor spreading. This resulted in relaxation of in-plane tensile stress (i.e. a pulse of relative compression) that caused internal plate deformation and enhanced mass wasting. This Aptian event' produced a strong, rift-wide reflection that is unconformably onlapped by post-rift sediments that were deposited as a stable sea-floor-spreading regime was established. Although previously considered to be a breakup unconformity associated with separation of continental crust, the event instead marks the final separation of the subcontinental mantle lithosphere. Our analysis indicates that interpretation of tectonic events in a non-volcanic rift must consider the rheology of the full thickness of the continental lithosphere, in addition to spatial and temporal changes in extension that may occur from segment to segment along the rift.

Description: Bouguer gravity anomalies together with deep seismic reflection and magnetic data on both sides of the North Atlantic are used to locate the hinge zones of the Flemish Cap and Galicia Bank within the Iberian and North American plates, regions across which there were abrupt changes in lithospheric extension. The characteristic shape and alignment of these hinge zones suggest that they were conjugate features generated during chrons M25M0 (Late JurassicEarly Aptian) around a distally located Euler pole of rotation. Rifting between Iberia and North America involved these two larger plates and the two smaller microplates the Flemish Cap and Galicia Bank microplates. The motion of the microplates, which were adjacent to Eurasia, was much more complex than those of the larger plates. The motion between the microplates from chron M25 or older to chron M0 was complicated by the fact that they remained attached to each other for most of the time when regions to the south were rifting apart. As a result, continental regions landward of these segments were subjected to extension that created the Orphan and Flemish Pass basins on the North American side and the Galicia Interior Basin on the Iberian side. By comparing the hinge zones delineated off Galicia Bank and Flemish Cap using the Bouguer anomalies, we were able to infer that Flemish Cap rotated approximately 43{degrees} relative to Galicia Bank and Iberia, and moved 200300 km SE with respect to North America. Such motions of Flemish Cap and Galicia Bank agree remarkably well with extensional episodes deduced from industry multichannel seismic reflection data acquired in the Orphan Basin. Normal fault orientations identified in the West Orphan Basin trend N020{degrees} and are approximately perpendicular to the flow lines of our proposed Flemish CapNorth American motion during the M25M0 period, which provides an independent constraint on our proposed kinematic model. Therefore, extensional events affected not only the Galicia BankFlemish Cap conjugate margins but also the Galicia Interior and Orphan Basins, and need to be taken into account in any assessment of the geological development of the Iberian and North American continental margins.