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 (70-76 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

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

[1] Mid-Cretaceous (Barremian-Turonian) plankton preserved in deep-sea marl, organic-rich shale, and pelagic carbonate hold an important record of how the marine biosphere responded to short- and long-term changes in the ocean-climate system. Oceanic anoxic events (OAEs) were short-lived episodes of organic carbon burial that are distinguished by their widespread distribution as discrete beds of black shale and/or pronounced carbon isotopic excursions. OAE1a in the early Aptian (∼120.5 Ma) and OAE2 at the Cenomanian/Turonian boundary (∼93.5 Ma) were global in their distribution and associated with heightened marine productivity. OAE1b spans the Aptian/Albian boundary (∼113–109 Ma) and represents a protracted interval of dysoxia with multiple discrete black shales across parts of Tethys (including Mexico), while OAE1d developed across eastern and western Tethys and in other locales during the latest Albian (∼99.5 Ma). Mineralized plankton experienced accelerated rates of speciation and extinction at or near the major Cretaceous OAEs, and strontium isotopic evidence suggests a possible link to times of rapid oceanic plateau formation and/or increased rates of ridge crest volcanism. Elevated levels of trace metals in OAE1a and OAE2 strata suggest that marine productivity may have been facilitated by increased availability of dissolved iron. The association of plankton turnover and carbon isotopic excursions with each of the major OAEs, despite the variable geographic distribution of black shale accumulation, points to widespread changes in the ocean-climate system. Ocean crust production and hydrothermal activity increased in the late Aptian. Faster spreading rates [and/or increased ridge length] drove a long-term (Albian–early Turonian) rise in sea level and CO2-induced global warming. Changes in ocean circulation, water column stratification, and nutrient partitioning lead to a reorganization of plankton community structure and widespread carbonate (chalk) deposition during the Late Cretaceous. We conclude that there were important linkages between submarine volcanism, plankton evolution, and the cycling of carbon through the marine biosphere.

Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid-Cretaceous

A chronology of the initiation dates of major continental flood basalt volcanism is established from published potassium-argon (K-Ar) and argon-argon (Ar-Ar) ages of basaltic rocks and related basic intrusions. The dating is therefore independent of the biostratigraphic and paleomagnetic time scales. Estimated errors of the initiation dates of the volcanic episodes determined from the distributions of the radiometric ages are, approximately, + or - 4 percent. There were 11 distinct episodes during the past 250 million years. Sometimes appearing in pairs, the episodes have occurred quasi-periodically with a mean cycle time of 32 + or - 1 (estimated error of the mean) million years. The initiation dates of the episodes are close to the estimated dates of mass extinctions of marine organisms. Showers of impacting comets may be the cause.

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Flood Basalt Volcanism During the Past 250 Million Years

The various geological signatures at Cretaceous/ Tertiary time including iridium and other associated elements, microspherules, and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism. The volatile emissions from this volcanism would lead to acid rain, reduction in the alkalinity and pH of the surface ocean, global atmospheric temperature changes, and ozone layer depletion. These environmental effects coupled with those related to the major sea level regression of the late Cretaceous provide the framework for an explanation of the selective nature of the observed extinction record.

Late Cretaceous and paroxysmal Cretaceous/Tertiary extinctions

Continental breakup is a highly variable process. Differences occur in the relative timing and extent of volcanism, uplift, and graben formation as well as in the mode and amount of continental extension before breakup. We propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends. Continental lithosphere is viewed as a composite material composed of two strong layers, one in the upper mantle and one in the middle crust. Finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust. This leads to early melt segregation and volcanism, margin uplift during the late stages of the extension process, and relatively narrow symmetrical extended margins. In contrast, failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle. This produces initial graben formation, cooling in the lower crust and upper mantle, and broad asymmetrical extended margins. Volcanism only occurs late in the process. Failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear. Thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup.

How preexisting weaknesses control the style of continental breakup

This report presents the results of a detailed correlation analysis which reveals a remarkable correspondence between variations in seafloor spreading rates and marine taxonomic diversity. The implications of this analysis are that the evolution of some biota was influenced by tectonic processes operative in the ocean basins. Such a conclusion suggests the need to seek a terrestrial explanation for extinction crises. An indirect link via tectonism, rise axis hydrothermal activity and the carbon dioxide cycle may provide a partial explanation.

A significant correlation between fluctuations in seafloor spreading rates and evolutionary pulsations

Scott and Balleny Islands are volcanic edifices in the Ross Sea. Scott Island rocks are phonolites similar to rocks found in the Hallett volcanic province, Antarctica. New samples from the Balleny Islands presented in this paper are undersaturated alkali lavas identical to those on other oceanic islands. It is suggested that intraplate stress within the Antarctic plate may be responsible for the Scott and Balleny Islands and other large seamount groups within the Ross Sea.

https://www.tandfonline.com/doi/pdf/10.1080/00288306.1982.10421508

Geology of Scott and Balleny Islands, Ross Sea, Antarctica, and morphology of adjacent seafloor

Petrological evidence suggesting a mantle plume beneath Iceland has been presented1,2. Schilling1 gives geochemical data which indicate that the incompatible elements TiO2, P2O5, K2O and La decrease regularly southwards from Iceland along the Reykjanes Ridge axis to about 61° N. The rare earth element pattern for Icelandic tholeiites shows a high normalised La/Sm ratio which steadily decreases to less than 1 along the ridge. The crystallisation index (FeO*/ MgO) and the ratio of pyroxene/plagioclase among the phenocrysts also follow this trend. Campsie et al.2 noted that tholeiites all the way from Iceland to the Charlie Gibbs Fracture Zone retain chemical coherence in their major elements (except SiO2) and belong to a single pyroxene-tholeiite magma type. Here we summarise petrological evidence for a Galapagos mantle plume, present new data on basalts from the rift valley of the Galapagos Ridge and discuss the origin and composition of the Cocos and Carnegie Ridges.

Chemistry of Tholeiites from the Galapagos Islands and Adjacent Ridges

An examination of the concentrations of the incompatible elements in tholeiites from the Kolbeinsey Ridge indicates that plume magmas have probably not overflowed into that area from Iceland or Jan Mayen Island. The results also show that the chemistry of plume magmas may be intimately related to regional geochemical patterns, and it is possible that Iceland represents a ‘weak spot’ or decompression focus, which causes complex flow patterns within and from the asthenosphere.

J. Campsie

Papers

A significant correlation between fluctuations in seafloor spreading rates and evolutionary pulsations

Geology of Scott and Balleny Islands, Ross Sea, Antarctica, and morphology of adjacent seafloor

Chemistry of Tholeiites from the Galapagos Islands and Adjacent Ridges

Dredged basalts from the mid-oceanic ridge north of Iceland

Dredged basalts from the western Nazca plate and the evolution of the East Pacific Rise