LARGE IGNEOUS PROVINCES - CRUSTAL STRUCTURE, DIMENSIONS, AND EXTERNALCONSEQUENCES

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basal ts and associated intrusive rocks, volcanic passive margins, oceanic p lateaus, submarine ridges, seamount groups, and ocean basin flood basa lts. Such provinces do not originate at ''normal'' seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and ana lyze dimensions, crustal structures, ages, and emplacement rates of re presentatives of the three major LIP categories: Ontong Java and Kergu elen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive ma rgins, and Deccan and Columbia River continental flood basalts. Crusta l thicknesses range from 20 to 40 km, and the lower crust is character ized by high (7.0-7.6 km s(-1)) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ont ong Java and Kerguelen, reveal short-lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the co ntemporaneous global production rate of the entire mid-ocean ridge sys tem. The major part of the North Atlantic volcanic province lies offsh ore and demonstrates that volcanic passive margins belong in the globa l LIP inventory. Deep crustal intrusive companions to continental floo d 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 a nd smaller ones developing in the upper mantle. This circulation coexi sts with convection associated with plate tectonics, a complicated the rmal structure, and at least four distinct geochemical/isotopic reserv oirs. LIPs episodically alter ocean basin, continental margin, and con tinental 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 enviro nment, scarce age and deep crustal data necessitate intensified effort s in seismic imaging and scientific drilling in a range of such featur es.