HR: 0800h
AN: T41B-1296    [Abstracts]
TI: Active Tectonics in the Dewa Hills, Northeast Japan: Late Quaternary Crustal Deformation and Horizontal Strain Rates Deduced from Deformed Fluvial Terraces and Simple Dislocation Model
AU: * Soeda, Y
EM: ysoeda@graduate.chiba-u.jp
AF: Graduate School of Science and Technology, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AU: Miyauchi, T
EM: tmiya@faculty.chiba-u.jp
AF: Department of Earth Science, Faculty of Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AB: The Dewa Hills in the Northeast Japan is a tectonically uplifted zone parallel to the direction of the Honshu arc, bounded by a 200-km-long Kitayuri Thrust Fault System (KTFS) at the western margin. The activity of KTFS resulting from crustal shortening related to subduction of the Pacific plate has affected the morpho-tectonogenesis of the Dewa Hills. We present such crustal shortening process and strain rates in the Dewa Hills, by analyzing the deformation pattern of topography and geology, and by the fault modeling. The Pleistocene fluvial terraces, a practical geomorphic marker for detecting the crustal deformation, are developed along some antecedent valleys that truncate the Dewa Hills. The M1 terrace along these rivers is identified as the last interglacial terrace by geomorphological and tephrochronological investigations. The obtained deformation pattern of M1 terrace shows the asymmetrical warping with wavelength of approximately 50 km, the maximum uplift rate of 0.5 mm/yr, and decreasing in the amount of uplift rate toward the inland basin area. This surface deformation is interpreted as a surface expression of the activity of the concealed KTFS in late Quaternary: the uplift of the Dewa Hills is accompanied by the growth of fault-bend fold. Applying the simple dislocation model (Mansinha and Smylie, 1971; Komatsubara, 1998) to the surface deformation pattern and previously published subsurface geological data, the required modeled fault geometry of KTFS is 8 to 11 degrees east dipping low-angle reverse fault penetrating the seismogenic layer, with a detachment and a ramp at a depth of approximately 6 km. Calculated net slip on this shallower ramp is significantly attenuated, compared with the deeper ramp below 6 km. This indicates that the slippage on the deeper part of the fault act important role for the geomorphic uplifting and the crustal shortening process. The horizontal strain rate in the Dewa Hills is estimated as 5.00 to 7.52 x 10^-8/yr. These values are as same as those derived from geological data in order. This suggests that high crustal shortening in the Dewa Hills has been occurring in late Quaternary and KTFS with high strain rates has morphotectonically promoted the construction of the Dewa Hills.
DE: 1145 Tephrochronology (8455)
DE: 8005 Folds and folding
DE: 8015 Local crustal structure
DE: 8107 Continental neotectonics (8002)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005