[S-CG49] 島弧の構造・進化・変形とプレート沈み込み作用

[SCG49-08] Structure and deformation in the overlying plates of Japanese arcs

★Invited Papers

*佐藤 比呂志^1,2 (1.東京大学地震研究所地震予知研究センター、2.静岡大学防災総合センター)

キーワード：上盤プレート、変形、深部地震探査、日本列島の島弧

The structure and deformation in Japanese arcs have been revealed last decades through densely distributed seismic, geodetic networks and deep seismic profiling. The M9.0 Tohoku-oki earthquake manifested the strong relationship between intraplate coupling and deformation of overlying plate. In the NE Japan arc, geological shortening deformation concentrated along the backarc continental crust. The shortening deformation was archived by reverse faulting of Miocene normal faults. Through the analysis of post M9.0 crustal deformation, Freed et al. (2017, EPSL) revealed the structure of overlying plate and wedge mantle, including the location of cold nose and thickness of overlying plate. Most of the geological shortening deformation occurs in the thin lithosphere (continental crust) between cold nose to the Sea of Japan oceanic crust underlaid by thick lithosphere. Through the recent seismic reflection profiling, the models of seismogenic source faults in and around the Sea of Japan were proposed (Sato et al., 2020: Abst. Seismol. Soc. Jpn). Reactivated reverse faults of Miocene normal faults shows moderate dip. Thrusts developed between oceanic and continental crust (No et al., 2014: EPSL) or along the margin of intra-continental backarc rift zones. Backarc continental rift zones are marked by thick post rift sediments and mafic lower crust. Thrusts along the backarc rift were produced by stress concentration along the rock boundary between mafic and felsic crust. Along the eastern part of Sea of Japan, the plate boundary between North American/Eurasia or Okhotsk/Amur is estimated. However, no suitable plate boundary faults were found by dense deep seismic profiling. Such plate convergence, if it exists, is accommodated in the wider zone of backarc in Northern Honshu. Through the saloon door opening of the Sea of Japan (Otofuji et al., 1985: Nature), counter clock-wise rotation of NE Japan was performed by strike-slip motion of the northern part of Itoigawa-Shizuoka tectonic line and Kanto tectonic line (Sato, 1994: GJR). High Vp lower crust underlaid along these intra-arc rift zones (Matsubara et al., 2017: JDR).

In SW Japan arc, basic structure of backarc is similar to NE Japan arc. Basic structure of rifted continental crust is marked by arc-parallel normal faults. The northward motion of Philippine Sea plate (PHS), mainly “hot” Shikoku basin” produced the large amount of NS-shortening in the backarc of SW Japan arc and formed the Shinji fold belts. This belt is marked by reverse faulting of older normal faults. Most of the NS-trending shortening terminated before Pliocene due to the northward subduction of PHS (Kimura et al., 2005: GSA Bull.). Change in motion of PHS from NNW to WNW, occurred at 1 Ma. This change created the reverse to right-lateral strike-slip faulting of Median Tectonic Line and asymmetric deformation of Izu collision zone. This westward motion of the PHS synchronized with main opening of the Okinawa trough associated with trench retreat and slab roll back.

Basic tectonic framework of Japanese arcs is strongly affected by the subduction of oceanic plates and slab behavior in the asthenosphere. Together with the change in coupling between the plate boundary, tectonic models of the overlying plate have to be constructed as a subduction system.