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 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 M25–M0 (Late Jurassic–Early 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° relative to Galicia Bank and Iberia, and moved 200–300 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° and are approximately perpendicular to the flow lines of our proposed Flemish Cap–North American motion during the M25–M0 period, which provides an independent constraint on our proposed kinematic model. Therefore, extensional events affected not only the Galicia Bank–Flemish 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.

Where plates converge, one-sided subduction generates two contrasting thermal environments in the subduction zone (low dT/dP ) and in the arc and subduction zone backarc or orogenic hinterland (high dT/dP ). This duality of thermal regimes is the hallmark of modern plate tectonics, which is imprinted in the ancient rock record as penecontemporaneous metamorphic belts of two contrasting types, one characterized by higher-pressure–lower-temperature metamorphism and the other characterized by higher-temperature–lower-pressure metamorphism. Granulite facies ultrahigh-temperature metamorphism (G-UHTM) is documented in the rock record predominantly from the Neoarchean to the Cambrian, although it may be inferred at depth in some younger Phanerozoic orogenic systems. Medium-temperature eclogite–high-pressure granulite metamorphism (E-HPGM) also is first recognized in the Neoarchean, although well-characterized examples are rare in the Neoarchean-to-Paleoproterozoic transition, and occurs at intervals throughout the Proterozoic and Paleozoic rock record. The first appearance of E-HPGM belts in the rock record registers a change in geodynamics that generated sites of lower heat flow than previously seen, inferred to be associated with subduction-to-collision orogenesis. The appearance of coeval G-UHTM belts in the rock record registers contemporary sites of high heat flow, inferred to be similar to modern arcs, abd backarcs, or orogenic hinterlands, where more extreme temperatures were imposed on crustal rocks than previously recorded. Blueschists first became evident in the Neoproterozoic rock record, and lawsonite blueschists, low-temperature eclogites (high-pressure metamorphism, HPM), and ultrahigh-pressure metamorphism (UHPM) characterized by coesite or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers low to intermediate apparent thermal gradients typically associated with modern subduction zones and the eduction of deeply subducted lithosphere, including the eduction of continental crust subducted during the early stage of the collision process in subduction-to-collision orogenesis. During the Phanerozoic, most UHPM belts have developed by closure of relatively short-lived ocean basins that opened due to rearrangement of the continental lithosphere within a continent-dominated hemisphere as Eurasia was formed from Rodinian orphans and joined with Gondwana in Pangea, and then due to successive closure of the Paleo-Tethys and Neo-Tethys Oceans as the East Gondwanan sector of Pangea began to fragment and disperse. The occurrence of both G-UHTM and E-HPGM belts since the Neoarchean manifests the onset of a “Proterozoic plate tectonics regime,” which evolved during a Neoproterozoic transition to the “modern plate tectonics regime” characterized by HPM-UHPM. The “Proterozoic plate tectonics regime” may have begun locally during the Mesoarchean to Neoarchean and may only have become global during the Neoarchean-to-Paleoproterozoic transition. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform. Extreme metamorphism occurs at times of amalgamation of continental lithosphere into supercratons (Mesoarchean to Neoarchean) and supercontinents (Paleoproterozoic to Phanerozoic), and along sutures due to the internal rearrangement of continental lithosphere within a continent-dominated hemisphere during the life of a supercontinent.