Abstract Two different types of ‘transitional lithosphere’ have been documented along magma-poor rifted margins. One consists of apparently subcontinental mantle that has been exhumed, brittlely deformed, and serpentinized during late stages of rifting. A second is thinned (<10 km) continental crust, which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases, it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter, and thus that transitional lithosphere has relatively limited width. Here three representative cases of transitional lithosphere are examined: one in the Newfoundland–Iberia rift and one at Goban Spur (both exhumed mantle), and one off the Angola–Congo margin (thin continental crust flanked seaward by apparently exhumed lower continental crust±exhumed mantle). Considering the geological and geophysical evidence, it appears that depth-dependent extension (riftward flow of weak lower continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed. Transitional lithosphere in this wide zone may consist of subcontinental mantle, lower continental crust or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional lithosphere ceases to be emplaced (i.e. ‘final break-up’ occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage break-up: first, the rupture of the brittle continental crust; and, second, the eventual separation of the ductile subcontinental lithosphere which is coincident with emplacement of normal oceanic crust. Well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpentinized, exhumed mantle, and such anomalies therefore are not diagnostic of oceanic crust. Where present, the anomalies can be helpful in interpreting and dating the rifting history.

Abstract The Orphan Basin formed during the Mesozoic intra-continental extension, continental breakup, and North Atlantic Ocean opening. New seismic data collected in the Orphan Basin, and regional potential field data, show a wide non-volcanic rift area with two successive rift zones characterized by greatly stretched continental crust. From both tectono-structural and petroleum potential points of views, the Orphan rifted area can be subdivided into an older East Orphan Basin situated in deep water (1,500-3,000m) and a younger West Orphan Basin situated in shallower water (1000–1,500m). Seismic stratigraphic relationships indicate that the first episode of rifting could be as old as the Late Triassic-Early Jurassic in the East Orphan Basin. A petroleum system including Kimmeridgian, marine source rocks is postulated for this basin. A second rifting stage, from latest Late Jurassic to Early Cretaceous, created the eastern part of the West Orphan Basin and re-mobilized basement blocks and older sedimentary features in the east Orphan Basin. A third extensional stage during mid-Cretaceous, probably coupled with Labrador Sea extension and opening, mostly affected the westernmost parts of the West Orphan Basin. A later extensional stage is postulated to have occurred in the early Tertiary, related to the initiation of a new rift between Greenland and northern Canada. The West Orphan Basin petroleum system should be anchored by Cretaceous or early Tertiary source rocks. The two main rift basins, east and West Orphan, are separated by a major crustal fault zone, the White Sail fault dipping eastward and affecting the entire upper crust. The N 020° linear, tilted faults blocks identified in the West Orphan Basin are perpendicular to the flow-lines of the herein proposed Flemish Cap motion during the M25-M0 period, giving an independent evidence of the validity of the Flemish Cap/North America reconstruction at chron M25 derived from magnetic and gravity data. On the M0 reconstruction, the east and West Orphan basins are located in front of the Porcupine basin and Rockall trough respectively. The Flemish Cap behaved as a large, monolithic continental block and rotated clockwise 43°, apparently moving more than 200 km to the southeast, while the upper crust of the east Orphan Basin has a = 2.5 average stretching from Late Triassic to early Tertiary. Regional seismic data suggest that trans-tensional movements, although hard to identify on reflection data, played an important role and continued until the Paleocene. Beyond the Orphan Knoll-Flemish Cap “outer ridge” lineament lies a true divergent margin basin showing little deformation within the sedimentary sequence and which overlies a relatively wide continent-ocean transition zone.

Abstract We present a new formulation of McKenzie’s simple uniform stretching model that is based on two reference levels, one near 3.6 km and the other near 7.8 km below sea level. In the absence of lithosphere, the asthenosphere would reach these levels if no oceanic crust were formed. The first level is for hot asthenosphere, the other is for asthenosphere cooled to thermal equilibrium. The instantaneous motion as well as the total vertical motion, produced by uniform stretching of the lithosphere, is expressed simply as a function of the elevation difference between the starting level and respectively the 3.6 and 7.8 km reference levels. In addition, we show that the behavior of the lithosphere under extensional strain is different above and below the 2.5 km-deep asthenosphere geoid. Below this level, oceanic accretion starts rapidly; above it, extensive thinning of the lithosphere produces subsidence until the asthenosphere geoid level is reached, enabling the asthenospheric material to break through to the surface. At low strain rate, pieces of the lower lithosphere may detach and sink in the asthenosphere. This process results in uplift and is taken into account in the formulation proposed.