Millard F. Coffin

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.

The separation of madagascar and Africa.

Identification of a sequence of east-west trending magnetic anomalies of Mesozoic age in the western Somali Basin helps define the position of Madagascar in the Gondwana reconstruction. The anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the Jurassic magnetic quiet zone. The motion of Madagascar relative to Africa was from the north and began in the middle Jurassic, about the same time as the initial breakup of Gondwanaland. Sea-floor spreading ceased when Madagascar assumed its present position in the Early Cretaceous.

Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean

Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998–99) and four from Legs 119 and 120 (1987–88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ~110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatile-rich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet–biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet–biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate.

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.

Impact origin for the greater Ontong Java Plateau

Abstract The ∼120 Ma Ontong Java Plateau and neighboring, contemporaneous Nauru, East Mariana, and (probably) Pigafetta basin flood basalts in the western equatorial Pacific Ocean comprise the Earth’s largest flood basalt province. Geophysical, geochemical, and geodynamic evidence from the province are difficult to reconcile with mantle plume models; absence of an obvious hotspot source or track, minor crustal uplift associated with emplacement, minor total subsidence compared with normal oceanic crust or other oceanic plateaus and submarine ridges, high degrees of melting at shallow, upper mantle depths, low water contents of basalts, enrichment of platinum group elements in basalts, and a ∼300 km deep, seismically slow mantle root are more consistent with the consequences of an impacting bolide. An object ∼20 km in diameter impacting relatively young (∼20 Myr) Pacific lithosphere and penetrating into the uppermost asthenosphere would have initiated massive decompression melting in the upper mantle, and may have resulted in emplacement of the greater Ontong Java Plateau.

Volcanism and continental break up: a global compilation of large igneous provinces

Abstract Large igneous provinces (LIPs) include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. In some cases transient episodes of voluminous magmatism are temporally and spatially related to continental break-up, e.g. North Atlantic Volcanic Province, Deccan Traps, Paraná-Etendeka basalts. In other cases, however, no relationships are apparent, e.g. Siberian flood basalts, Columbia River flood basalts. Herein we review LIPs worldwide in order to better understand their relationship to the break-up and separation of lithospheric plates. The two most voluminous episodes known of basaltic magmatism not associated with the creation of ‘normal’ oceanic crust, the emplacements of the Ontong Java and Kerguelen plateaus, do not appear to be linked to continental break-up. Volcanic passive margins have now been identified on the edges of many continents, and are clearly related to continental break-up and separation. They cannot always, however, be tied to continental flood basalts. Ocean basin flood basalts and seamount groups are not commonly related to continental break-up. In most instances submarine ridges show temporal and spatial relationships with continental flood basalt provinces or oceanic plateaus. Observational data suggest that existing models do not adequately explain all LIPs; we suggest that a thermally and chemically heterogeneous asthenosphere, occasionally penetrated by deep mantle plumes, can account for their origin.

Neotectonics of the Macquarie Ridge Complex, Australia-Pacific plate boundary

New marine geophysical data along the Macquarie Ridge Complex, the Australia-Pacific plate boundary south of New Zealand, illuminate regional neotectonics. We identify tectonic spreading fabric and fracture zones and precisely locate the Australia-Pacific plate boundary along the Macquarie Ridge Complex. We interpret a ∼5–10 km wide Macquarie Fault Zone between the two plates along a bathymetrie high that extends nearly the entire length of the Australia-Pacific plate boundary south of New Zealand. We conclude that this is the active Australia-Pacific strike-slip plate boundary. Arcuate fracture zones become asymptotic as they approach the plate boundary. A broad zone of less intense deformation associated with the plate boundary extends ∼50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike-slip deformation, although teleseismic evidence overwhelmingly indicates strike-slip motion along the entire surveyed boundary today. The McDougall and southernmost Puysegur segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia-Pacific crust in the region. Our conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex addresses the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.

Present-day Plate Boundary Digital Data Compilation

The PLATES Project compiled digital data representing the present-day plate boundaries. This report includes the digital data in three formats: GMT, KMZ and ArcGIS shapefiles.

Provenance of Proterozoic garnet-biotite gneiss recovered from Elan Bank, Kerguelen Plateau, southern Indian Ocean

At Elan Bank of the Kerguelen Plateau in the southeast Indian Ocean, Leg 183 of the Ocean Drilling Program recovered clasts of garnet-biotite gneiss in a fluvial conglomerate intercalated with basalt flows. U-Pb and Pb-Pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 Ma, which is much older than the surrounding Indian Ocean seafloor. These dates show that old continental crust resides in the shallow crust of the oceanic Kerguelen Plateau and that the breakup of Gondwana dispersed continental fragments into the nascent Indian Ocean lithosphere.