OPHIOLITIC MELANGE: NOT A CHAOTIC CASE BUT A FAULT ZONE

Two representaitive ophiolitic melanges of forearc settings, one is the Eocene-Miocene Mineoka ophiolite in Boso Peninsula, south of Tokyo, and the other is the Cretaceous Ankara ophiolitic melange, east of Ankara, Turkey, were compared, and the compositions, arrangements, structures, and configurations were known to be similar each other not to be simple melanges nor chaotic but fault zones of several deformation stages. The former is composed of MORB/BABB, IAT and WPB, and pelagic and terrigenous sedimentary rock blocks looked to be involved within sheared serpentinite but actually to be in fault contact, and the serpentinite (mainly harzburgite) is massive, only sheared and brecciated along the fault contacts. The latter is of similar compositions with many calcareous beds, making fault zones with serpentinite that is sheared only on the fault contacts. The fault zones have common arrangements inside characterized by Riedel and thrust shears, mixed with ophiolitic rock-derived clastic sediments which indicate the obduction stage. The basaltic rocks in the Mineoka have at least three series of deformations; the first occurs around the spreading ridges (or first magma emplacement stage) in normal and strike-slip fault regime with hydrothermal activities, the second is characterized by oblique thrusts during obduction, and the third is later displacement into fault zones as a forearc sliver strike slip. The term melange only means undifferentiated origin or mechanism, and does not extend beyond the field descriptive term. The real emplacement mechanism should be identified after careful mapping, structural analysis and critical discussion.