Most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins, both of which underwent only small amounts of crustal thinning. In this paper, we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended, deep Iberia‐Newfoundland margins. Our results show that continental breakup is a complex process distributed in time and space. On the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension. During a first phase, dated as Tithonian to Barremian (145–128 Ma), deformation is related to exhumation of mantle rocks; basins become younger oceanward, and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries. A second phase, dated as latest Aptian (112 Ma), overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs. The observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins.

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-fl...

SUMMARY Two tomographic methods for assessing velocity models obtained from wide‐angle seismic traveltime data are presented through four case studies. The modelling/inversion of wide‐angle traveltimes usually involves some aspects that are quite subjective. For example: (1) identifying and including later phases that are often difficult to pick within the seismic coda, (2) assigning specific layers to arrivals, (3) incorporating pre‐conceived structure not specifically required by the data and (4) selecting a model parametrization. These steps are applied to maximize model constraint and minimize model non‐uniqueness. However, these steps may cause the overall approach to appear ad hoc, and thereby diminish the credibility of the final model. The effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide‐angle data set: the first‐arrival times. For data sets with Moho reflections, the tomographic velocity model can be used to invert the PmP times for a minimum‐structure Moho. In this way, crustal velocity and Moho models can be obtained that require the least amount of subjective input, and the model structure that is required by the wide‐angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data. The tomographic models are not intended to supersede the preferred models, since the latter model is typically better resolved and more interpretable. This form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models. Four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above. This was followed by conventional first‐arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth, minimum‐structure models. The case studies are from the SE Canadian Cordillera across the Rocky Mountain Trench, central India across the Narmada‐Son lineament, the Iberia margin across the Galicia Bank, and the central Chilean margin across the Valparaiso Basin and a subducting seamount. These case studies span the range of modern wide‐angle experiments and data sets in terms of shot–receiver spacing, marine and land acquisition, lateral heterogeneity of the study area, and availability of wide‐angle reflections and coincident near‐vertical reflection data. The results are surprising given the amount of structure in the smooth, tomographically derived models that is consistent with the more subjectively derived models. The results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide‐angle traveltime data.

[1] We have studied a nonvolcanic margin, the West Iberia margin, to understand how the mechanisms of thinning evolve with increasing extension. We present a coincident prestack depth-migrated seismic section and a wide-angle profile across a Mesozoic abandoned rift, the Galicia Interior Basin (GIB). The data show that the basin is asymmetric, with major faults dipping to the east. The velocity structure at both basin flanks is different, suggesting that the basin formed along a Paleozoic terrain boundary. The ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension. At the rift flanks (stretching factor, b 2) the ratio is fairly constant, indicating that stretching of upper and lower crust was uniform. Toward the center of the basin (b $ 3.5-5.5), fault-block size decreases as the crust thins and faults reach progressively deeper crustal levels, indicating a switch from ductile to brittle behavior of the lower crust. At b ! 3.5, faults exhume lower crustal rocks to shallow levels, creating an excess of lower crust within their footwalls. We infer that initially, extension occurred by large-scale uniform pure shear but as extension increased, it switched to simple shear along deep penetrating faults as most of the crust was brittle. The predominant brittle deformation might have driven small-scale flow ( 40 km) of the deepest crust to accommodate fault offsets, resulting in a smooth Moho topography. The GIB might provide a type example of nonvolcanic rifting of cold and thin crust.

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