Olav Eldholm

Large igneous provinces: Crustal structure, dimensions, and external consequences

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0-7.6 km s?1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short-lived pulses of increased global production; Ontong Java’s rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

Evolution of the Norwegian-Greenland Sea

Geological and geophysical data collected aboard R/V Vema during five summer cruises in the period 1966 to 1973 have been used to investigate the geological history and evolution of the Norwegian-Greenland Sea. These data were combined with earlier data to establish the location of spreading axes (active as well as extinct), the age of the ocean floor from magnetic anomalies, and the locations and azimuths of fracture zones. The details of the spreading history are then established quantitatively in terms of poles and rates of rotation. Reconstructions have been made to locate the relative positions of Norway and Greenland at various times since the opening, and the implications of these reconstructions are discussed here.

North Atlantic volcanic margins: Dimensions and production rates

Early Tertiary lithospheric breakup between Eurasia and Greenland was accompanied by a transient (∼3 m.y.) igneous event emplacing both the onshore flood basalts of the North Atlantic Volcanic Province (NAVP) and huge extrusive complexes along the continent-ocean transition on the rifted continental margins. Seismic data show that volcanic margins extend >2600 km along the early Eocene plate boundary, in places underlain by high-velocity (7.2–7.7 km/s) lower crustal bodies. Quantitative calculations of NAVP dimensions, considered minimum estimates, reveal an areal extent of 1.3×106 km2 and a volume of flood basalts of 1.8×106 km3, yielding a mean eruption rate of 0.6 km3/yr or 2.4 km3/yr if two-thirds of the basalts were emplaced within 0.5 m.y. The total crustal volume is 6.6×106 km3, resulting in a mean crustal accretion rate of 2.2 km3/yr. Thus NAVP ranks among the worlds larger igneous provinces if the volcanic margins are considered. The velocity structure of the expanded crust seaward of the continent-ocean boundary differs from standard oceanic and continental crustal models. Based on seismic velocities this “volcanic margin” crust can be divided into three units of which the upper unit corresponds to basaltic extrusives. The regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust, including the high-velocity lower crust which extends some distance landward of the continent-ocean boundary, was emplaced during and subsequent to breakup. The volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling, probably reflecting the interaction of a mantle plume and a lithosphere already extending.

Continental Margin off Norway: A Geophysical Study

Geophysical investigations consisting of gravity, magnetic, depth sounding, seismic reflection and refraction measurements were made aboard R/V Vema on the continental margin off Norway. Utilizing these and earlier data in the area, maps showing bathymetry, free-air gravity, magnetic residual total intensity, seismic refraction results, total sediment thickness and thickness of Cenozoic sediments have been constructed. The data are also presented as profiles across the margin. The Voring Plateau is underlain by a buried escarpment; the basement is shallow on the seaward side and deep on the landward side. A similar marginal escarpment, the Faroe–Shetland, exists farther south. These escarpments mark the site of the Tertiary opening of the Norwegian Sea. Seaward, Tertiary sediments overlie a basement generated by sea-floor spreading. Landward, a thick sequence of sediments that may be as old as Paleozoic overlies a continental basement. The magnetic quiet zone on the landward side of the escarpment is attributed to the continental nature of the basement. A nearly continuous belt of positive gravity and magnetic anomalies that exists just landward of the edge of the shelf is attributed primarily to intrabasement density contrasts in rocks that are probably Precambrian in age. It extends from northwest Scotland to the Lofoten–Vesteralen islands. The continental margin off Norway formed an epicontinental sea continuous with the North Sea in which a large amount of sedimentation kept pace with subsidence—a phenomenon which perhaps started in the late Paleozoic. The thickness of pre-Cenozoic sediments exceeds 6 km in some areas, but has a relative minimum over the belt of high density rocks of Precambrian age, which presumably underwent the least relative subsidence. We suggest that the opening of the Norwegian Sea at the marginal escarpments is associated with subsidence of the continental crust between the escarpments and the shelf where the high-density belt acts as a hinge line and accounts for the existence of the shelf break. The subsided area is characterized by a regional free-air gravity low. The marginal Voring Plateau Escarpment formed at the opening of the Norwegian Sea served to dam the Tertiary sediments and develop the V0ring Plateau. The Norwegian Channel is shown not to be of tectonic origin. The Tertiary basin of the North Sea continues northward under the continental margin off Norway.

Continent-ocean transition at the western Barents Sea/Svalbard continental margin

The change in crustal type at the western Barents Sea/Svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the Greenland Sea. Regionally, the margin is composed of two large shear zones and a central rifted-margin segment. Local transtension and transpression at the plate boundary caused the early Cenozoic tectonism in Svalbard and the western Barents Sea, and might explain the prominent marginal gravity and velocity anomalies.

NE Atlantic continental rifting and volcanic margin formation

Abstract Deep seismic data from the Hatton-Rockall region, the mid-Norway margin and the SW Barents Sea provide images of the crustal structure that make it possible to estimate the relative amounts of crustal thinning for the Late Jurassic-Cretaceous and Maastrichtian-Paleocene NE Atlantic rift episodes. In addition, plate reconstructions illustrate the relative movements between Eurasia and Greenland back to Mid-Jurassic time. The NE Atlantic rift system developed as a result of a series of rift episodes from the Caledonian orogeny to early Tertiary time. The Late Palaeozoic rifting is poorly constrained, particularly with respect to timing. However, rifted basin geometries, inferred to be of this age, are observed at depth in seismic data on the flanks of the younger rift structures. Intra-continental rifting in Late Jurassic-Cretaceous times caused c. 50–70 km of crustal extension and subsequent Cretaceous basin subsidence from the Rockall Trough-North Sea areas in the south, to the SW Barents Sea in the north. In late Early to early Late Cretaceous times, new rifting occurred in the Rockall Trough and Labrador Sea associated with the northward propagation of North Atlantic sea-floor spreading. When sea-floor spreading was approached in the Labrador Sea the Rockall rift apparently became extinct. The final NE Atlantic rift episode was initiated near the Campanian-Maastrichtian boundary, lasted until continental separation near the Paleocene-Eocene transition, and caused c. 140 km extension. The late syn-rift and the earliest sea-floor spreading periods were affected by widespread igneous activity across a c. 300 km wide zone along the rifted plate boundary. The deep seismic data provide lower-crustal structural geometries that represent boundary conditions for a better mapping and understanding of the extensional thinning of the crust. The crustal geometries question extension estimates previously made from basin subsidence analysis, and aid in the definition of bodies of magmatic underplating beneath the outer volcanic margins.

New aerogeophysical study of the Eurasia Basin and Lomonosov Ridge: Implications for basin development

In 1998 and 1999, new aerogeophysical surveys of the Arctic Ocean9s Eurasia Basin produced the first collocated gravity and magnetic measurements over the western half of the basin. These data increase the density and extend the coverage of the U.S. Navy aeromagnetic data from the 1970s. The new data reveal prominent bends in the isochrons that provide solid geometrical constraints for plate reconstructions. Tentative identification of anomaly 25 in the Eurasia Basin links early basin opening to spreading in the Labrador Sea before the locus of spreading in the North Atlantic shifted to the Norwegian-Greenland Sea. With the opening of the Labrador Sea, Greenland began ∼200 km of northward movement relative to North America and eventually collided with Svalbard, Ellesmere Island, and the nascent Eurasia ocean basin. Both gravity and magnetic data sets reconstructed to times prior to chron 13 show a prominent linear anomaly oriented orthogonal to the spreading center and immediately north of the Yermak Plateau and Morris Jesup Rise. This anomaly may mark the locus of shortening and possibly subduction as Greenland collided with the nascent Eurasia Basin and impinged upon the southern Gakkel Ridge. This collision may have contributed to volcanism on the Morris Jesup Rise. By chron 13, Greenland had ended its northward motion and had become fixed to North America, and the plateau north of Greenland had rifted apart to become the Morris Jesup Rise and the Yermak Plateau.

Scratching the surface: Estimating dimensions of large igneous provinces

A study of five major basaltic provinces, including oceanic plateaus, volcanic passive margins, and continental flood basalts, shows that they are voluminous constructions of extrusive igneous rock underlain by intrusive rock. Crustal thickness ranges from 20 to 40 km, and lower crust is characterized by high (7.0-7.6 km/s) seismic velocities. Volumes and emplacement rates derived for two oceanic plateaus, the Ontong Java and Kerguelen-Broken Ridge, reveal short-lived pulses of increased global crustal production and suggest an origin involving the lower mantle. The Ontong Java rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system. Despite the importance off large igneous provinces in studies of mantle dynamics and the global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

Crustal structure of the Ontong Java Plateau: modeling of new gravity and existing seismic data

Seismic refraction and gravity-based crustal thickness estimates of the Ontong Java oceanic plateau, the Earths largest igneous province, differ by as much as 18 km. In an attempt to reconcile this difference we have evaluated available seismic velocity data and developed a layered crustal model which includes (1) a linear increase in velocity with depth in the Cenozoic sediments and the uppermost extrusive basement and (2) a reinterpretation of deep crustal and Moho arrivals in some deep refraction profiles. Previously, Moho had commonly been interpreted from later arrivals and in some cases constrained by precritical arrivals. However, if first arrivals at distal offsets are interpreted as Moho refractions, the maximum depth to Moho is reduced by about 10 km. Two-dimensional gravity modeling along two transects from well-determined oceanic crust in the Nauru Basin across the central On-tong Java Plateau to the Lyra Basin, based on the reinterpreted crustal model, is regionally consistent with satellite altimetry derived and shipboard gravity fields yielding a 8.0 km/s Moho velocity at a depth of ?32 km under the central plateau. The crust features a thick oceanic, three-layer igneous crust comprising an extrusive upper crust, a 6.1 km/s middle crust and a ?15 km thick 7.1 km/s lower crust. The total Ontong Java Plateau crustal volume is calculated at 44.4 × 106 km3 and 56.7 × 106 km3 for off- and on-ridge emplacement settings, respectively. On the basis of velocities and densities we interpret the lower crust on the plateau to consist of ponded and fractionated primary picritic melts, which due to deformation and/or fluid invasion may have recrystallized to granulite facies mineral assemblages. The melts were emplaced during lithospheric breakthrough of a mantle plume in an oceanic, near-ridge plate tectonic setting.

Environmental impact of volcanic margin formation

Late rift stage uplift and subsequent massive, transient volcanism during breakup of rifted volcanic continental margins constrain paleoenvironments by modifying basin geometry and the composition of the atmosphere, hydrosphere and thus biosphere on regional and global scales. The early Tertiary North Atlantic breakup history shows that lava emplacement was accompanied by regional ashfalls, and that extrusive complexes influenced Paleogene oceanic and continental margin circulation and sedimentation. Temporal correspondence with the terminal Paleocene deep-sea extinction event and the earliest Eocene greenhouse suggests a global impact, possibly by enhanced atmospheric CO 2 levels, leading to polar warming and thereby changing patterns of deep-water formation. In this context, transient subaerial volcanism at continental margins should be considered with the much discussed continental flood basalt provinces and oceanic plateaus.