Abstract:
The Newfoundland-Iberia 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 150-180 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 Aptian-Albian boundary, which is also some 15 million years younger than the oldest proposed oceanic crust (anomaly M5-M3) 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 Triassic-earliest Jurassic and Late Jurassic-Early 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 Madeira-Tore Rise. We propose that the rising asthenosphere breached the subcontinental mantle lithosphere in latest Aptian-earliest 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.