Tomotaka Iwata

By Yasuto Itoh and Tomotaka Iwata

A geological survey for the Late Cretaceous Izumi Group distributed on the Median Tectonic Line (MTL) active fault system in the central part of southwestern Japan has revealed varied deformation styles. Among the confined deformation zones found in the western and central parts of the study area, some are located far from the active trace of the MTL (Negoro Fault), at distances of up to 300–350 m. Such kink zones may have been generated during a contraction phase of the MTL from the end of the Pliocene to the early Pleistocene. We identified clear active foldings in a narrow zone sandwiched between a north dextral and a south reverse active fault. Western and eastern upheavals of the crustal sliver show ridge and domal active morphologies, respectively. Structural analysis was extended to the north of the MTL, where the Izumi Group has suffered multiphase deformation since the Cretaceous. The phase stripping method was introduced to extract the neotectonic trend, which successfully delineated complicated deformation zones related to the morphological divergence of the MTL active fault system.

Part of the book: Evolutionary Models of Convergent Margins

By Yasuto Itoh, Tomotaka Iwata and Keiji Takemura

The subsurface morphology of an arc-bisecting tectonic zone has been unraveled by means of well-organized seismic investigation with the aid of borehole stratigraphic control. The Median Tectonic Line (MTL) active fault system in southwestern Japan, one of the world’s largest intraplate transcurrent faults, is driven by the recent oblique subduction of the Philippine Sea Plate. Six tied seismic profiles covering the mountainous range, the southern foothill of which is truncated by the MTL, were used to visualize the Quaternary basins on both feet of the Izumi Mountains. North- and east-trending basement deformation was confirmed on the northern and southern sides of the watershed, respectively; this deformation reflects the spatial diversity in tectonic stress. Seismic data on the southern Izumi flank revealed a low-angle fault parallel to the MTL active fault system; this fault may be interpreted as a dormant structure that developed from 6 to 2 Ma under the intermittent increases of the compressive regime. A kink zone in the upthrown block of the thrust was identified on seismic profiles and continuously traced through field geological survey. This zone confirms the prevailing contractional phase related to the transient convergence mode of the oceanic plate.

Part of the book: Evolutionary Models of Convergent Margins

By Yasuto Itoh, Paul F. Green, Keiji Takemura and Tomotaka Iwata

Fission track (FT) thermochronology was applied to the Late Cretaceous turbidite sandstones of the Izumi Group adjacent to the Median Tectonic Line active fault system in southwest Japan. Apatite FT analyses revealed the following three stages of cooling (uplift) events: 95–78 Ma (Cenomanian–Campanian) from >130°C, 74–46 Ma (Campanian–middle Eocene) from approximately 100°C, and 27–7 Ma (late Oligocene–late Miocene) from approximately 70°C. By contrast, zircon FT analysis indicated cooling from >300°C at ca. 70 Ma. Apparent discrepancies between the cooling initiation times obtained using the two analytical methods indicate the distinct provenances of tuffaceous sandstones of the Izumi Group. The second episode is likely related to regional exhumation events on the eastern Eurasian margin. The latest event, which terminated by the end of the Miocene, appears to have been manifested in the strong deformation of the arc under a compressive stress provoked by the resumed subduction of the Philippine Sea Plate.

Part of the book: Evolutionary Models of Convergent Margins