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Ontong Java and Kerguelen Plateaux: Cretaceous Icelands?

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Ontong Java and Kerguelen Plateaux: Cretaceous Icelands? Journal of the Geological Society, London, Vol. 152, 1995, pp. 1047-1052, 4 figs. Printed in Northern Ireland Ontong Java and Kerguelen Plateaux: Cretaceous Icelands? M. F. COFFIN & L.M. GAHAGAN Institute for Geophysics, The University of Texas at Austin, 8701 North Mopac Expressway, Austin, Texas 78759-8397, USA Abstract: Together with Iceland, the two giant oceanic plateaux, Ontong Java in the western Pacific and Kerguelen/Broken Ridge in the Indian Ocean, are accumulations of mafic igneous rock which were not formed by 'normal' seafloor spreading. We compare published geochronological, crustal structure, and subsidence results with tectonic fabric highlighted in new satellite-derived free-air gravity data from the three igneous provinces, and conclude that existing evidence weighs lightly against the Ontong Java and Kerguelen plateaux originating at a seafloor spreading centre. Keywords: Iceland, Ontong Java Plateau, Kerguelen Plateau, plumes, hot spots. The two giant oceanic plateaux, Ontong Java in the western Age constraints Pacific, and Kerguelen in the south-central Indian Ocean (Fig. 1), and Iceland are among the best-studied examples of The vast bulk of crust in the ocean basins is dated using large-scale mafic magmatism not resulting solely from magnetic anomalies created by the interplay between the 'normal' seafloor spreading. Analogues on the continents, seafloor spreading process and the alternating polarity of the continental flood basalts, are demonstrably not created by Earth's magnetic field. Mesozoic and Cenozoic marine seafloor spreading, although controversy persists as to magnetic anomalies, summarized globally by Cande et al. whether or not lithospheric extension must precede their (1989), are most commonly tied to geological time through emplacement. The question exists, were the largest oceanic radiometric dating of basalt samples obtained from crust plateaux created at a seafloor spreading centre (e.g. near displaying a particular anomaly (e.g. Cande & Kent 1992, active transform faults), as is the case with Iceland, or were 1995; Gradstein et al. 1994). In some cases, notably Iceland, they created away from active plate boundaries, as is an active seafloor spreading centre can be traced on land Hawaii? We address this question using available age, using a combination of marine and aeromagnetic data (e.g. crustal structure, subsidence and tectonic fabric data, Nunns et al. 1983). Because one-third of the Iceland igneous following a brief description of the plateaux' physical province (Fig. 2) lies above sea level, radiometric dating has characteristics. been possible on many samples. It has confirmed that Iceland has been created by anomalously voluminous, subaerial seafloor spreading, characterized by an age The plateaux progression from 0 Ma along the locus of crustal generation to as much as 15 Ma (McDougall et al. 1984) away from that The Ontong Java Plateau, in the western equatorial Pacific zone (and still above sea level). Multiple and shifting (Fig. 1), encompasses an area of 1 860000km 2 (Coffin & spreading axes, laterally extensive lava flows, numerous Eldholm 1994). Although neighbouring flood basalts in the central volcanoes, erosion, and alteration, however, East Mariana, Nauru, and Pigafetta basins, as well as those contribute to a rather complex tectonic history. of the Manihiki Plateau, may represent part of the same The advent of the 4°Ar/3~Ar dating technique has igneous event that created the Ontong Java Plateau, we brought needed resolution to dating oceanic basalts, and has consider only the Ontong Java Plateau sensu stricto in this had a particularly large impact on oceanic plateaux in comparison with the Kerguelen and Iceland plateaux. The particular. Igneous basement samples from the Ontong Java Kerguelen Plateau and Broken Ridge, which were separated and Kerguelen plateaux, however, are sparse (Figs 3 & 4). by seafloor spreading in Eocene time (Houtz et ai. 1977), Datable igneous rock has only been recovered offshore from together cover an area of 2300000km 2 in the southern three scientific drill sites on the Ontong Java Plateau (289, Indian Ocean (Coffin & Eldholm 1994). Igneous basement 803, 807). Onshore sampling of igneous basement is possible of Broken Ridge has not been penetrated by drilling, and on the southern part of the plateau that has been uplifted to the relationship of rocks dredged from the rifted southern form Malaita and Santa Isabel islands. Mahoney et al. (1993) flank of Broken Ridge to the Ridge's underlying igneous have reported that 4°Ar/39Ar dates of igneous basement at basement is unknown (Duncan 1991). Therefore, in this DSDP Site 289, ODP Site 807, and on Malaita Island are all comparison we consider only the Kerguelen Plateau, which 120-122Ma. Some Santa Isabel basalts have also given covers an area of 1 780000km 2. Iceland, in the North dates of 120-122 Ma. Igneous basement at ODP Site 803 Atlantic approximately equidistant from Greenland and N and other basalts on Santa Isabel Island, however, have Europe (Fig. 1), lies astride the Mid-Atlantic Ridge. That yielded dates of 88-90Ma (Bercovici & Mahoney 1994). part of the Iceland Plateau above sea level covers The geographic distribution of these two dates of basaltic c. 100 000 km2; if submerged portions are included, on the volcanism is not easily reconciled with simple models of age basis of bathymetry, the area totals c. 300 000 km 2. transgressive volcanism along a spreading axis, and has led 1047 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/6/1047/4893212/jgs_152_006_1047.pdf by guest on 30 September 2021 1048 M.F. COFFIN & L. M. GAHAGAN ~,Nauru Basin 0o t "~ .) Manihiki ~~ ~ ' Broken ~OntonqPlateauJa/a "~ '~'L~~ i • " Ridge ~'~.~."• 4~Platea0/u / PLATES/UTIG Kerquelen~ " / 30 ° S oO Pla_-teau / ~ 18 Fig. 1. Ontong Java (Ontong Java, Manihiki, Nauru, Pigafetta, and East Mariana), Kerguelen (Kerguelen Plateau, Broken Ridge), and Iceland mafic igneous provinces. Outlines of Ontong Java and Kerguelen in this and Figs 3 and 4 are after Coffin & Eldholm (1994); Iceland in this and Fig. 2 after the General Bathyrnetric Chart of the Oceans (IHO/IOC/CHS 1984). Bercovici & Mahoney (1994) to propose two volcanic last decade. For example, the gravity field over Iceland episodes caused at 120-122 Ma by an initial plume head and (Fig. 2; Sandwell et al. 1994) clearly shows how NE-SW- at 88-90Ma by the subsequent surfacing of a detached trending, ridge-parallel spreading fabric is continuous plume tail. from the North Atlantic ocean basin NE across the Iceland Igneous basement of the Kerguelen Plateau has been Plateau to the coast of SW Iceland. This, of course, con- drilled and dredged, and in places cores of sediment and a firms what we know from magnetic anomalies and radio- basalt flow provide minimum age estimates for basement metric dating of onshore basalts. The situation to the NE (Fig. 4). A portion of the Plateau has been interpreted to be of Iceland is not as well displayed on the free-air gravity Cenozoic in age (Coffin & Eldholm 1994), and is not field, but is similar in that symmetric, ridge-parallel discussed further here. On the main, older part of the spreading fabric can be traced from the ocean basin SW to Plateau, Davies et al. (1989) obtained a date of 114 Ma using practically the shoreline. the K-Ar method on a dredged basalt. Whitechurch et al. Given that the Ontong Java and Kerguelen plateaux are (1992) 'determined a wide variety of dates employing Cretaceous in age, one would expect that any possible 4°Ar/39Ar techniques on whole rock ODP basement association with relict seafloor spreading axes would be samples, yet concluded that the bulk of the plateau formed much more subdued than is the case with Iceland and the at c. ll0Ma, on the basis of their most reliable date, active Mid-Atlantic spreading axes to the NE and SW. 109.5 Ma, and the Davies et al. date. Pringle et al. (1994) Relict spreading axes and ridge jumps have been identified have redated the ODP basement samples (Fig. 4), in some places in the ocean basins (Mammerickx & determining ages of c. 110 Ma for sites 750, 749, and 738. Sandwell 1986). In the cases of Ontong Java and Kerguelen, ODP Site 747 basalts have yielded a date of 85 Ma, similar however, marine magnetic anomalies identified to date (Figs to Whitechurch et al.'s date for the flow encountered at Site 3 & 4) provide no clues as to whether the plateaux formed 748. These latter dates are similar to that obtained from at a spreading centre or intra-plate. This is in large part due sediment at the bottom of a piston core on the NE flank of to the fact that a large part of the Ontong Java Plateau the Kerguelen Plateau (Frt~hlich & Wicquart 1989). formed near the start of, and that the Kerguelen Plateau formed during, the Cretaceous superchron, a c. 35Ma period over which the Earth's geomagnetic field remained in Tectonic fabric dominently normal polarity. Great improvement in our knowledge of the tectonic fabric The NE flank of the Ontong Java Plateau is of the seafloor has come from the free-air gravity field over characterized by marked NE-SW trending lineations of the oceans derived from satellite altimetry data over the unknown origin: possibilities include seafloor spreading Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/6/1047/4893212/jgs_152_006_1047.pdf by guest on 30 September 2021 ONTONG JAVA AND KERGUELEN PLATEAUX 1049 fabric (fracture zones and ridge-parallel faults and abyssal hills) and folding of the crust. The lineations intersect 330" 335" 340" 345" 350" interpreted magnetic isochrons Mll-M14 and postulated fracture zones obliquely, suggesting that if the lineations represent fracture zones or seafloor-spreading fabric, existing interpretations require modification. Longer wave- length, NW-SE-trending lineations are observed on the northern plateau; these may represent folds related to collision of the Ontong Java Plateau with the Solomons arc.
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