HR: 0800h
AN: T41C-1225    [Abstracts]
TI: Knocker Problem Revisited: Plutonic and Metamorphic Blocks in Mineoka Ophiolite of Oblique Subduction Boundary, Japan
AU: * Mori, R
EM: ryoken@arsia.geo.tsukuba.ac.jp
AF: Master Program in Science and Technology, Univeristy of Tsukuba, Tsukuba, Iba 305-8572 Japan
AU: Ogawa, Y
EM: yogawa@arsia.geo.tsukuba.ac.jp
AF: Doctoral Program in Earth Evolution Sciences, University of Tsukuba, Tsukuba, Iba 305-8572 Japan
AB: In Boso Peninsula on Sagami trough, Japan, ophiolitic rocks called the Mineoka Ophiolite form an east-west fault zone along a forearc sliver fault parallel to the present oblique subduction boundary. The fault zone is composed of serpentinite, picrite, alkali basalts (WPB), tholeiitic basalts (MORB, BABB or IAT), chert, and limestone, most are of Eocene age, and are associated with Miocene volcaniclastic or serpentine-bearing sedimentary rocks. Plutonic rocks such as diorite and gabbro (island arc chemistry), and metamorphic rocks (mafic with siliceous and psammitic) such as hornblende schist and amphibolite are included. We observed some critical contacts between these rocks to know the processes and mechanisms of emplacement, and further discussed the tectonic implication for the knocker problem in the forearc ophiolite fault zone. These knockers are a few m to 100 m in scale, involved as isolating exotic or in situ blocks, being cut by faults in various directions with strong deformation into breccia. A diorite knocker at Yamada (Yamada Knocker, 30 _~ 15 _~ 15 m) is in fault contact with serpentinite on the north which is thermally metamorphosed and tectonically sheared by diorite intrusion and dislocation. Two system shear zones of cataclasite to mylonite are remarkable. The first stage shear penetrates the rocks, but the second is only around the boundary. These planes are of normal-fault sense with strike-slip component, and are plotted on a small circle of a vertical pole. We interpreted that the first stage brecciation occurred during obducting stage of the ophiolite, and the second faulting during the rearrangement of the fault zone at the present setting as a forearc sliver fault. Such two stage deformation is common in all the blocks, including metamorphic, plutonic and basaltic rocks, suggesting the role of the knocker deformation is strongly control the oblique plate boundary tectonics.
DE: 9355 Pacific Ocean
DE: 8150 Plate boundary--general (3040)
DE: 8158 Plate motions--present and recent (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting