HR: 0800h
AN: T11B-0373    [Abstracts]
TI: Uplift and Horizontal Shortening Rate of the Northeast Japan Arc During the Past 150 kyr, Estimated From Fluvial/Marine Terrace Data
AU: * Tajikara, M
EM: tajikara@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Ikeda, Y
EM: ikeda@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AB: The northeast (NE) Japan arc is located along the NE Japan subduction zone, where the Pacific plate is subducting beneath the North American plate at a rate of 8-9 cm/yr. Compressional deformation of the NE Japan arc is attributed to the subduction of the Pacific plate. We estimated the rates and spatial distribution of uplift in the middle part of the NE Japan arc on a geologic time scale (since about 150 ka). The height difference between river profiles of Marine Isotope Stage (MIS) 1 and MIS 5e (or MIS 2 and MIS 6) are used as an indicator of uplift during those two stages. This method is based on the model in which elevation of river is controlled mainly by climate and sea levels, and in which the longitudinal river profiles formed in similar sea level and climatic conditions have a similar geometry. River profiles of MIS 2, 5e, 6 are constructed from fluvial terraces. Obtained spatial distribution of uplift rates indicates that the deformation of the NE Japan arc consists of the deformation with short wavelength (10-50 km) and the deformation with long wavelength (150-200 km). The short-wavelength deformation is attributed to activity of active faults, since the points where uplift rates vary abruptly correspond to the surface traces of known active faults. The causes of the long-wavelength deformation are pointed out as follows: (1) isostatic uplifts due to crustal thickening, (2) isostatic uplifts due to denudation, and (3) isostatic subsidence due to load of volcanic deposits. Crustal thickening is caused by crustal shortening and magmatic underplating. Average uplift rates over the inland area of the NE Japan arc are 0.24-0.32 mm/yr. By subtracting isostatic uplift due to denudation and isostatic subsidence due to load of volcanic deposits from average uplift rate over the NE Japan arc, we obtain the rate of isostatic uplift rate due to crustal thickening at 0.21-0.31 mm/yr. Crustal thickening rate can be calculated from this value at 1.3-1.7 mm/yr. If we ignore the effect of magmatic process, and if we assume that arc-parallel deformation is negligibly small, and that crustal material are incompressible (no change in volume), horizontal shortening and strain rate of the NE Japan arc are calculated at 6.6-8.5 mm/yr and 4.2-5.6 x 10^-8/yr, respectively. Horizontal strain rate obtained in this study is in good agreement with those estimated by other geologic observations on different time scales; those values are on an order of 10^-8/yr. This implies that horizontal strain rate of the NE Japan arc has been constant in Pliocene and Quaternary time. Furthermore, obtained crustal shortening rate indicates that only 10 % of the plate convergence has been accommodated within the NE Japan arc as permanent (plastic) deformation of the arc crust.
DE: 1825 Geomorphology: fluvial (1625)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005