HR: 0800h
AN: T41C-1226    [Abstracts]
TI: Structural Analysis of Broken Formation in the Shimanto Accretionary Prism Implies Oblique Subduction Tectonics
AU: * ENDO, R
EM: goodfat@arsia.geo.tsukuba.ac.jp
AF: Tsukuba University, tennoudai1-1-1, Tsukuba, 305-8572 Japan
AU: ANMA, R
EM: anma@arsia.geo.tsukuba.ac.jp
AF: Tsukuba University, tennoudai1-1-1, Tsukuba, 305-8572 Japan
AB: In order to understand strain distribution inside the strongly folded and deformed broken formation of the Cretaceous-Paleogene Shimanto accretionary prism Yakushima Island, Kyushu, Japan, we carry out detailed mapping of outcrop features based on high-resolution aero-photographs and AMS measurement on oriented core specimens. The so-called melanges, composed of folded and faulted turbidites, are classified into zone 1 and 2 in this area. The former contains blocks that preserve original bedding and development of layer-sub-parallel high strain shear zones in otherwise chaotic sequence, whereas the latter develops rootless folds and sheath folds. In zone 1, maximum principal axis of AMS fabric (Kmax) is disposed vertically inside vertically dipping bedding planes and Kmin normal to the bedding. Kmax orientations are sub-parallel to axes of vertical meso-scale folds inside zone 1. Zone 2 is highly deformed, but Kmax direction is also vertical as zone 1. Axes of rootless folds were plotted on a great circle on a stereonet projection suggesting development of sheath folds. Possibilities of these folds developed due to submarine landslide were ruled out because quartz veins normal to flattening surface of sandstone lenses and fan-shaped in the hinge zones were developed and in some cases silicified totally replacing original depositional texture of SS lenses. We attribute this to deformation under moderate T-P conditions and under influence of slica-saturated liquid. High-resolution mapping elucidates relationship between both zones; Zone 1 is distributed in upper part of thrust anticline and zone 2 beneath it. Since rootless folds in zone 2 rotated during high-strain deformation, we used relationship between vertical axes of cylindrical folds and AMS orientation of its hinge zones to estimate shape of strain developed during melange formation. In our analysis, Kmax was sub-parallel to the vertical fold axes and this implies maximum stretching direction was subparallel to axes of thrust anticline. We attribute this stretching parallel to fold axes to transpressional deformation during oblique subduction.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8005 Folds and folding
DE: 8157 Plate motions--past (3040)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting