HR: 0800h
AN: T11C-0403    [Abstracts]
TI: Smash tectonics of the Mineoka ophiolite belt in the Boso TTT-type triple junction margin
AU: * Mori, R
EM: ryoken@arsia.geo.tsukuba.ac.jp
AF: University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Iba 305-8572 Japan
AU: Ogawa, Y F
EM: yogawa@arsia.geo.tsukuba.ac.jp
AF: University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Iba 305-8572 Japan
AU: Kurosawa, M
EM: kurosawa@arsia.geo.tsukuba.ac.jp
AF: University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Iba 305-8572 Japan
AU: Tsunogae, T
EM: tsunogae@arsia.geo.tsukuba.ac.jp
AF: University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Iba 305-8572 Japan
AU: Hirano, N
EM: nhirano@f2.dion.ne.jp
AF: Tokyo Institute of Technology, Ookayama 2-12-1, Meguroku, Tok 152-8551 Japan
AB: Quite unique history and tectonics were verified from the Mineoka ophiolite belt in the Boso TTT-type triple junction margin, NW Pacific corner, by stratigraphy, chemistry, dating, and particularly deformation of the basaltic, plutonic and metamorphic blocks within a serpentinite-bearing fault zone. First, Cretaceous rocks (chert and basalt) were found from the previously known Paleogene ophiolitic rocks and their Paleogene-Miocene pelagic cover. Pillow basalts, ranging from MORB-BABB to IAT,_@might be come back to Early Cretaceous. If the ophiolitic rocks belong to one single plate, it would be a Creataceous-Paelogene oceanic plate or island arc (possibly forearc) body. Next, the deformations are various with at least three major stages, consisting of the first normal-strike-slip faulting under ductile conditions, the second strike-slip (dextral) faulting with thrust-component under deeper ductile conditions. The third stage is large scale-mixture by strike-slip (dextral) faulting under shallower brittle conditions into breccia (zeolite facies) incorporated with various rocks ranging from the Cretaceous to Paleogene ophiolitic rocks, Miocene alkali basalts, Cretaceous-Miocene chert-limestone, and middle Miocene terrigenous clastic sediments with serpentinite fragments and continent-island arc derived materials. This final stage deformation occurred in a transpressional regime, forming Riedel shear system. It is still continuing to the present forearc sliver fault zone. This phenomenon has a key to understanding the uplift and exhumation process of the deeper metamorphic and plutonic rocks in the convergent margin. Those deformed rocks are covered with undeformed andesitic pumice fall deposits of middle Miocene age which has basalt and diorite xenoliths. All such components in the Mineoka belt are not distributed outside but restricted only in the belt between the Miocene forearc basin and accretionary prism, strongly indicating the very specific history and tectonics. The best candidate for the original Mineoka ophiolite is the Izu forearc seamount or plateau, like the Hahajima seamount off the Bonin Islands. Such specific forearc part with schistose amphibolite-facies metamorphic rocks was experienced by various deformations and sedimentations into the final TTT-triple junction smash tectonics, which brings all such materials into a single forearc sliver fault system seen at present in the sandwiched area between the Izu and Honshu forearcs.
DE: 3039 Oceanic transform and fracture zone processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3042 Ophiolites (8140)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 8106 Continental margins: transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005