HR: 09:05h
AN: T21E-05    [Abstracts]
TI: The Puzzle of the Petrology of Oceanic Plateaux
AU: * Foulger, G R
EM: g.r.foulger@durham.ac.uk
AF: University of Durham, Dept. Earth Sciences, Science Laboratories, South Rd., Durham, DH1 3LE United Kingdom
AB: The Iceland oceanic plateau is the best-studied in the world regarding crustal structure because a 450 x 350 km portion of it is exposed above sea level and facilitates experiments of much higher quality than can currently be achieved at sea. Numerous studies have been performed there over the last 3-4 decades, including state-of-the-art explosion and passive seismic experiments that have collected data on digital, three-component instruments. Diverse processing methods have been applied to these data including receiver functions, tomography, waveform modelling, wide-angle reflection analyses and mapping of Vp/Vs and attenuation. Additional information is available from magnetotelluric work on land in Iceland and marine seismic experiments on the flanking Greenland-Iceland and Iceland-Faeroe aseismic ridges. Nevertheless, despite the huge amount of information available from this plateau (or perhaps because of it) there remains a fundamental, unsolved puzzle regarding the petrology of the crust. The thickness of material with crust-like seismic wave speeds varies from ~ 20 km to ~ 40 km and appears to be typically ~ 30 km over much of the marine and subaerial plateau. Isostatic considerations predict that the elevation of Iceland should be ~ 4 km above sea level for normal crustal and mantle densities. However, instead the smoothed dome of Iceland is typically only ~ 1 km above sea level. From this it may be concluded that the density contrast between the ~ 20-km-thick Icelandic lower crust and the mantle beneath is only ~ 100 kg/m³ instead of the ~ 300 kg/m³ that would be expected for a gabbroic lower crust and peridotitic upper mantle. No reasonable petrologies have yet been suggested that can explain the observations, leaving unresolved the question of how much of the seismic crustal thickness is derived from melt and whether some may be mantle containing unextracted melt or cumulates containing entrained mantle. As a result, and despite the near-exhaustion of currently available geophysical probing techniques, we remain frustratingly ignorant of the melt productivity of the Iceland "hot spot", a parameter that is key to understanding melt extraction process at melting anomalies. Other plateaux with comparable seismic structures include the Ontong Java Plateau, the Madagasgar Ridge and the Kerguelan Islands. Does the study of obducted material hold the key to understanding the petrology of deep parts of the crust beneath oceanic plateaux?
UR: http://www.mantleplumes.org/
DE: 0930 Oceanic structures
DE: 0935 Seismic methods (3025, 7294)
DE: 3037 Oceanic hotspots and intraplate volcanism
DE: 3038 Oceanic plateaus and microcontinents
DE: 7220 Oceanic crust
SC: Tectonophysics [T]
MN: Fall Meeting 2005