HR: 0830h
AN: V21D-0554    [PDF]
TI: Drilling the Largest Oceanic Detachment in the World: the Godzilla Mullion in the Parece Vela Rift
AU: * Ohara, Y
EM: ohara@jodc.go.jp
AF: Hydrographic and Oceanographic Department of Japan, Tsukiji, Chuo-ku, Tokyo, 104-0045 Japan
AU: Okino, K
EM:
AF: Ocean Research Institute, University of Tokyo, Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Snow, J E
EM:
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, D-55020 Germany
AU: Salisbury, M
EM:
AF: Geological Survey of Canada (Atlantic), Dartmouth, Nova Scotia, B2Y 4A2 Canada
AB: Recently discovered megamullions at slow-spreading ridges have been interpreted as exhumed footwalls of oceanic detachment faults in magma-starved ridge environments. These features are believed to result from extreme tectonic extension, and thus they are ideal to study processes of tectonic extension in oceanic lithosphere. In addition to tectonic issues, these features provide excellent tectonic windows to the lower crust and uppermost mantle. Drilling megamullion structures allows us to study tectonic processes, serpentinization and magnetization at shallow depth and mantle composition and fabrics at a deeper level. The largest megamullion structure in the world is the Godzilla Mullion in the extinct Parece Vela backarc basin, Philippine Sea. Recent reconnaissance studies by our group have revealed the following unique characteristics of this structure: (1) The Godzilla Mullion is huge, $\sim$ 10 times larger in area than megamullions identified so far on the Mid-Atlantic Ridge (MAR). (2) The latest KR03-01 cruise dredged mantle peridotites (see Snow et al., this session) from entire length of the Godzilla Mullion (over 125 km), suggesting that an ultramafic exposure of some $\sim$ 7000 km2. (3) The Godzilla Mullion develops along the full along-strike length of the spreading segment. In the MAR, the megamullions normally develop at inside corners of ridge-transform intersections. (4) Peridotites from the Parece Vela Basin include very fertile lherzolites with spinel Cr\# [Cr/(Cr+Al)] = $\sim$ 0.17. (5) The Godzilla Mullion was formed in a relatively fast intermediate-spreading environment with 7.0 cm/y full-rate. We may expect a relatively high magmatic budget in the fast/intermediate-spreading Parece Vela Basin, since a high magmatic budget is generally expected for a fast-spreading ridge. However, the characteristics noted above instead indicate a small degree of mantle melting. We suggest that these features may result from the characteristic ridge-transform geometry of short first-order segments sandwiched by closely-spaced fracture zones. We propose to drill the Godzilla Mullion using the IODP's non-riser platform, employing an offset drilling strategy. Although there may be uniqueness inherited from the characteristic ridge-transform geometry, we value the following two major merits of drilling the Godzilla Mullion: (1) The Godzilla Mullion is the nominally fastest megamullion in the world, providing a necessary bridge in observations of oceanic lithospheric processes between the existing and planned fast (i.e., Hess Deep) and slow spreading (i.e., MARK and 15-20 FZ) drill holes. (2) The length of the Godzilla Mullion ($\sim$ 125 km) suggests that an unusually deep oceanic as yet unsampled mantle section may be exposed.
DE: 1025 Composition of the mantle
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting