Quantifying crustal back-arc extension and shortening in the Northeast Japan arc

Thursday, 18 December 2014
Shinsuke Okada, IRIDeS, Tohoku Univ., Sendai, Japan, Yasutaka Ikeda, University of Tokyo, Earth and Planetary Science, Bunkyo-ku, Japan and Toshifumi Imaizumi, Tohoku University, Sendai, Japan
The Northeast Japan (NEJ) arc has experienced strong extension in Early to Middle Miocene time and contractive deformation since Pliocene time. We determined the amounts of the Miocene extension and Pliocene contraction along four transects across the back-arc region of NEJ arc by area-balancing restoration using seismic reflection, gravity, and surface geologic data. We found that the Miocene extension is as much as 31–56 km while the Pliocene-Quaternary contraction is only 10–15 km. The style of the Miocene extension in the back-arc region of NEJ is highly asymmetric; the zone of concentrated extension (mainly along the Japan Sea margin) is characterized by a breakaway fault on the west, a rollover basement anticline on the east, and abnormally deep (∼10 km) basins and strongly rotated fault blocks in between, suggesting the existence of a large-scale detachment fault at a mid-crustal level beneath the extended zone. We suggest that the Pliocene-Quaternary contractive surface deformation in the back-arc region of NEJ, which is represented by fault-bend folds and fault-propagation folds in rift basins, resulted principally from positive tectonic inversion using the same detachment fault at a mid-crustal depth.

Although the strong Pliocene-Quaternary contraction has occurred mainly in the NEJ’s back-arc, week deformation has occurred also in the NEJ’s forearc, suggesting that the underlying detachment fault has propagated eastward beneath the forearc. To reveal more details of crustal extension and shorting of NEJ arc, it is necessary to estimate the amounts of extension and shortening of the inland area and forearc side precisely. In this presentation, we will demonstrate the tectonic evolution in the back-arc region of Northeast Japan since Miocene time, with recent results of seismic reflection surveys in the inland area and forearc side of NEJ.