Anthony G. Doré

Principal tectonic events in the evolution of the northwest European Atlantic margin

The Atlantic margin of the Norwegian, Faeroese, British and Irish sectors encompasses numerous basins which vary in character, but are related in terms of their evolution as part of a single passive margin. Lineament analysis of the margin shows a predominance of NE–SW, N–S and NW–SE trends, mainly reflecting Mesozoic–Cenozoic extensional faulting. Some major Precambrian and Caledonian structures, principally steeply-dipping shears, were opportunistically reactivated according to the prevalent stress pattern. The extensional history of the margin spanned a c. 350 Ma interval between the close of the Caledonian orogeny and early Eocene break-up. Episodes of Permo-Triassic, (mainly late) Jurassic, Early Cretaceous, ‘middle’ Cretaceous and latest Cretaceous–Early Eocene age can be distinguished from one another in space and time. The anomalous length of the total period of extension prior to continental separation is partly explained by step-wise lateral offsets of the crustal thinning axes towards the line of eventual break-up. The picture is, however, complicated by some changes in extensional style and direction. These include mosaic-like fragmentation of Pangea in the Permo-Triassic, the imposition of more systematic E–W extension by Jurassic times, and the change to NW–SE extension focused on the present margin in the Early Cretaceous (probably Hauterivian). The resulting structural configuration reflects the overprinting of a complex network of Jurassic and older basins by a continuous NE–SW chain of deep Cretaceous-Cenozoic basins. An extensional pulse of latest Cretaceous to earliest Eocene age (best observed in the Norwegian Sea) with extensive basaltic volcanism led to continental break-up at approximately 53 Ma. The margin was structurally modified by some important events postdating the Early Eocene. On breakup, the background stress field changed from extension to mild SE-directed compression, and widespread inversion structures formed in the thick Cretaceous-Cenozoic depocentres. The inversions can best be explained by ridge-push from the adjacent spreading centres, but could also be linked to Tethyan closure events and changes in the North Atlantic spreading vector. Post-break-up extension of the North Atlantic passive margins has been reported in the western Barents Sea, Jan Mayen and East Greenland and (for the first time here) in the northern Voring Basin. We propose that these areas were linked by a single extensional pulse induced by the change to a more ESE-directed relative plate motion in the Oligocene-Miocene. Major uplift and exhumation of peripheral landmasses and inboard basins took place at intervals throughout the Cenozoic. Initial uplift can be attributed to pre-break-up rifting and post-break-up compression, but the most significant event took place in the Plio-Pleistocene and was intimately associated with glacial erosion and isostatic adjustment through repeated glaciations and interglacials. The regional scale of this event and its significance for exploration is widely under-estimated.

Cenozoic compressional structures on the NE Atlantic margin; nature, origin and potential significance for hydrocarbon exploration

Compressional structures of Cenozoic age are ubiquitous features of the NE Atlantic margin between the western Barents Sea and offshore western Ireland. The structural suite includes simple domes or anticlines, reverse faults and broad-scale inversions. Our analysis focuses on a recently delineated group of structures in the Norwegian Sea, although these are placed in the context of similar features on the Barents margin, West of Shetlands, on the Faroes and their surrounding shelf, and in the Rockall Trough. Some (although not all) of the compressional anticlines were formed at the sites of pre-existing Cretaceous-Palaeocene depocentres. They show a multi-phase growth history. In the Norwegian Sea, particularly important phases occurred in the middle Eocene to early Oligocene and in the Miocene. We interpret the formation of these structures as a natural outcome of the transition to sea-floor spreading that occurred in the early Eocene. From this time, extremely thick sedimentary successions that had accumulated during some 300 million years of extensional tectonics were subjected to mild compression. The overall compressive stress field can be explained in terms of spreading in the adjacent ocean (ridge push) and by the distant effects of Alpine tectonics. In a plate-wide sense these effects can be regarded as two sides of the same coin. The origin of the Norwegian Sea structures is most easily visualized in terms of ridge push. NW-SE transfer zones, characteristic of the entire margin, are strongly implicated in these tectonics. A kinematic model is described that links significant structuring with a change in the spreading direction of Oligocene-Miocene age (35-20 Ma, A13-6). The compressional structures are mainly observed by their effect on sediments and volcanics of Cretaceous and Cenozoic age. They are frequently expressed in the present-day sea-floor relief, and in the case of the Faroe islands are probably responsible for present subaerial exposure. From the point of view of hydrocarbon exploration, the Cenozoic compressive anticlines have obvious potential as fourway dip closures or as components of structural-stratigraphic traps. The NW-SE fractures, orientated parallel or sub-parallel to the maximum horizontal stress direction, were probably periodically open for fluid flow from the time of NE Atlantic opening and onwards. They may therefore have facilitated migration from deeper source rocks or remigration from pre-existing hydrocarbon accumulations.

Hyperextension, serpentinization, and weakening: A new paradigm for rifted margin compressional deformation

The Early Eocene magma-rich northeast Atlantic rifted margins contain a large number of pre-breakup and post-breakup compressional structures, located in abandoned Early Cretaceous hyperextended basins with crustal stretching factors of 3–4 or more. The deformation both predates and postdates the magma-rich breakup. The hyperextended basins are often underlain by high-velocity lower crustal bodies, which we argue represent partially serpentinized upper mantle. Long-lived lithospheric weakening and proneness to deformation is proposed to relate to crustal hyperextension, probably enhanced by mantle hydration.

Cenozoic exhumation of the southern British Isles

Rocks that crop out across southern Britain were exhumed from depths of as much as 2.5 km during Cenozoic time. This has been widely attributed to Paleocene regional uplift resulting from igneous underplating related to the Iceland mantle plume. Our compilation of paleothermal and compaction data reveals spatial and temporal patterns of exhumation showing little correspondence with the postulated influence of underplating, instead being dominated by kilometer-scale variations across Cenozoic compressional structures, which in several basins are demonstrably of Neogene age. We propose that crustal compression, due to plate boundary forces transmitted into the plate interior, was the major cause of Cenozoic uplift in southern Britain, witnessing a high strength crust in western Europe.

The NE Atlantic margin; implications of late Mesozoic and Cenozoic events for hydrocarbon prospectivity

The study takes in the entire NE Atlantic margin (NEAM) but emphasizes the sparsely drilled More and Voring Basins. A network of Permo-Triassic and Jurassic basins was strongly overprinted by younger extensional episodes. At least three phases are probable--early Cretaceous, mid-Cretaceous and latest Cretaceous-early Eocene--between the late Jurassic and break-up. Substantial thicknesses of Cretaceous and Cenozoic strata along the margin have focused exploration interest on the late Cretaceous and Paleocene intervals as easily drillable targets. Reservoirs within these intervals were deposited as gravity-driven sand incursions into an overwhelmingly mud-prone environment. Sand pulses of Albian-Coniacian, Santonian-early Campanian and Paleocene age occur widely, and can be tied to the tectonic episodes. Vertical migration of hydrocarbons from known Jurassic source rocks is proven west of Shetlands, but in many parts of the margin remigration from intermediate reservoirs would be required to charge the shallower plays. Failing this, prospectivity will hinge on the presence of frontier source rocks, with best possibilities at Barremian, Aptian-Albian, Cenomanian-Turonian and Paleocene levels. Potential hydrocarbon traps were formed by pre-breakup extensional faulting and by post-breakup compression.

Exhumation of the North Atlantic margin: introduction and background

Since consolidation during the Caledonian and Variscan orogenies, NW Europe has undergone repeated episodes of exhumation (the exposure of formerly buried rocks) as a result of such factors as post-orogenic unroofing, rift-shoulder uplift, hotspot activity, compressive tectonics, eustatic sea-level change, glaciation and isostatic readjustment. Modern measurement techniques, such as apatite fission-track analysis, have helped to establish useful denudation chronologies for this entire time span. However, the main observational legacy of exhumation around the North Atlantic is preserved in the comparatively young (Mesozoic and Cenozoic) geological record of this region. This is clearly reflected by the unifying theme of this volume, which documents evidence for the widespread uplift and emergence of large sections of the North Atlantic margin in Cenozoic time. All students of NW European geology are aware of the compelling palaeogeographical evidence for the transition at the end of the Cretaceous from shelf seas and low-relief landmasses to an area dominated by highlands and newly emergent landmasses, flanked by shelves dominated by rejuvenated clastic deposition. Similarly, it is also widely known that the highlands of Norway and Scotland do not represent the original Caledonian mountain range but must be instead a product of late emergence or uplift. The Cenozoic uplift of Fennoscandia in particular has a long history of study. It is arguably one of the oldest debates in the history of systematic geology and featured prominently in Lyell’s Principles of Geology (Lyell 1830–1875). All of this early work was, of course, based on onshore observations. By the late

Transform margins of the Arctic: a synthesis and re-evaluation

Abstract Transform-margin development around the Arctic Ocean is a predictable geometric outcome of multi-stage spreading of a small, confined ocean under radically changing plate vectors. Recognition of several transform-margin stages in the development of the Arctic Ocean enables predictions to be made regarding tectonic styles and petroleum systems. The De Geer margin, connecting the Eurasia Basin (the younger Arctic Ocean) and the NE Atlantic during the Cenozoic, is the best known example. It is dextral, multi-component, features transtension and transpression, is implicated in microcontinent release, and thus bears close comparison with the Equatorial Shear Zone. In the older Arctic Ocean, the Amerasia Basin, Early Cretaceous counterclockwise rotation around a pole in the Canadian Mackenzie Delta was accommodated by a terminal transform. We argue on geometric grounds that this dislocation may have occurred at the Canada Basin margin rather than along the more distal Lomonosov Ridge, and review evidence that elements of the old transform margin were detached by the Makarov–Podvodnikov opening and accommodated within the Alpha–Mendeleev Ridge. More controversial is the proposal of transform along the Laptev–East Siberian margin. We regard an element of transform motion as the best solution to accommodating Eurasia and Makarov–Podvodnikov Basin opening, and have incorporated it into a three-stage plate kinematic model for Cretaceous–Cenozoic Arctic Ocean opening, involving the Canada Basin rotational opening at 125–80 Ma, the Makarov–Povodnikov Basin opening at 80–60 Ma normal to the previous motion and a Eurasia Basin stage from 55 Ma to present. We suggest that all three opening phases were accompanied by transform motion, with the right-lateral sense being dominant. The limited data along the Laptev–East Siberian margin are consistent with transform-margin geometry and kinematic indicators, and these ideas will be tested as more data become available over less explored parts of the Arctic, such as the Laptev–East Siberia–Chukchi margin.

The Mesozoic evolution of the southern North Atlantic region and its relationship to basin development in the south Porcupine Basin, offshore Ireland

Abstract The Mesozoic history of a number of Atlantic borderland sedimentary basins can be related to the early opening history of the southern North Atlantic Ocean. Regional tectonic controls such as plate motion vectors and the pre-existing tectonic grain had an important role in basin development and are expressed as local tectonostratigraphic events. The evolving palaeogeographies for the region are demonstrated in a series of maps based on computer-generated plate reconstructions. The Porcupine Basin, centrally located in the study area, lay close to the intersection of three plate boundaries that separated Eurasia from North America and controlled opening of the Bay of Biscay. The south Porcupine Basin, where there is relatively poor data control, is considered in the context of broader platetectonic controls, which were also responsible for the development of contiguous and better understood basins during Mesozoic time. This approach provides new insight into the structural evolution and likely facies development in the south Porcupine Basin, allowing broad inferences for petroleum prospectivity to be made. Initial Permo-Triassic fault-controlled extension led to continental deposition, which, if associated with aeolian and/or fluvial reservoir rocks, will mostly be too deep to be prospective. Thermal subsidence during Early Jurassic time was associated with flooding and fine-grained clastic deposition with anticipated moderate source rock potential. Regional uplift of the northern Porcupine area during Mid-Jurassic time forced shorelines and shelves southwards and the south Porcupine Basin could contain good reservoir quality sandstones and possible waxy deltaic-type source rocks of this age. In Late Jurassic time, major crustal extension took place with potential for reservoir and source rocks in locally expanded footwall successions. Further extensional faulting occurred in earliest Cretaceous (Neocomian) time with further synrift plays possible at this level. Growth of the Porcupine Median Volcanic Ridge is attributed to Barremian-Aptian time and related to continuing extension associated with a northwesterly arm of a triple junction positioned to the south of the Porcupine area. Strong subsidence of the basin centre during this time will have a significant impact on source rock maturation and flank trap geometries in the south Porcupine Basin.

Repeated inversion and collapse in the Late Cretaceous–Cenozoic northern Vøring Basin, offshore Norway

The Norwegian Atlantic margin, although frequently described as passive, has seen several significant and highly variable deformation events prior to and after early Cenozoic break-up. This chronology is strongly exemplified in the northern Vøring Basin, where deformation resulted in significant vertical motions, including deep erosion and sediment reworking. Post-break-up compressional deformation is well documented in the NE Atlantic margins, and is represented in the north Vøring Basin by the Vema and Naglfar domes. A prominent Maastrichtian–Paleocene pre-break-up phase of compression inverted the northern prolongation of the latest Turonian Vigrid Syncline. This syncline was the fairway for the approximately 1 km-thick Santonian–Campanian Nise Formation sandstone, shed from NE Greenland and/or the western Barents Sea margin. The inversion focused on the Vigrid Syncline axis, forming an anticline here referred to as the Vema–Nyk Anticline. The anticline may have been a major trap but was breached by erosion prior to collapse due to Late Paleocene extension. The remnant eastern half of the anticline is the Nyk High. The associated flanking syncline, the Någrind Syncline, also remains preserved. The collapsed side of the anticline is the Hel Graben, which itself was inverted in the Middle Miocene time forming the Naglfar and Vema domes. More speculatively, the development of the Vigrid Syncline and its bounding structural highs, the Gjallar Ridge and Utgard High, may also represent folds, marking the onset of compressional buckling in the mid-Norwegian–NE Greenland rift system. The repeated compressional deformation, as well as the extensional collapse, was focused on the area subjected to Early Cretaceous hyperextension. Compressional buckling under relatively low stress levels is proposed to have been due to significant lithosphere weakening caused by the hyperextension, whereby both high attenuation of the crystalline crust and serpentinization of the upper mantle contribute to the weakening. The Late Cenozoic compression post-dated the hyperextension by approximately 110 Ma, which suggests that the weakening is long-lived and that lithosphere has not been strengthened significantly through time.

Fixity of the Iceland “hotspot” on the Mid-Atlantic Ridge: Observational evidence, mechanisms, and implications for Atlantic volcanic margins

The Iceland anomaly has been attributed to a deeply rooted and fixed mantle plume, and Early Tertiary magmatism in the North Atlantic Igneous Province (NAIP) has commonly been interpreted to relate to an ancient expression of the same plume. We challenge these concepts. A major problem with attributing the Iceland anomaly to a fixed plume is the lack of evidence for a hotspot track. Although the GreenlandFaeroe Ridge has been suggested to be the hotspot track, its symmetric and continuous construction instead suggests in situ development on the plate boundary. Magmatism in the NAIP occurred in two phases, distributed in approximately perpendicular belts. The first phase (ca. 62–58 Ma) occurred along a north-west belt through the British Volcanic Province to west Greenland. We relate this phase to a transient and failed rift, intermediate in time and space between seafloor spreading in the Labrador Sea and the northeast Atlantic. The second phase (ca. 56–53 Ma) followed the incipient northeast Atlantic plate boundary. Both magmatic phases can therefore be associated with plate tectonics. Likewise, the north Atlantic–Arctic breakup can be explained as a natural outcome of plate tectonics and lithospheric strength distribution. We follow other recent research in suggesting that the voluminous magmatism during NAIP phase 2 is related to reactivation and opening along the Caledonian orogen. Specifically, we point to a close correspondence between the reactivated orogen and the north Atlantic volcanic passive margins, and suggest that the extreme magmatism could stem from the melting of eclogitic material, either residing in remnants of the CaledonianAppalachian orogenic root or within a delaminated root. Extending this idea, we postulate as a testable hypothesis that volcanic margins are the natural products of the Wilson Cycle (i.e., opening of sutures). We have tested the hypothesis on the north, central, and south Atlantic Ocean and have found a broad correlation between volcanic margin segments and reopened Late Neoproterozoic–Phanerozoic fold belts.