Tertiary evolution of the Shimanto belt (Japan): A large‐scale collision in Early Miocene

Abstract

To decipher the Miocene evolution of the Shimanto belt of southwestern Japan, structural and paleothermal studies were carried out in the western area of Shikoku Island. All units constituting the belt, both in its Cretaceous and Tertiary domains, are in average strongly dipping to the NW or SE, while shortening directions deduced from fault kinematics are consistently orientated NNW‐SSE. Peak paleotemperatures estimated with Raman spectra of organic matter increase strongly across the southern, Tertiary portion of the belt, in tandem with the development of a steeply dipping metamorphic cleavage. Near the southern tip of Ashizuri Peninsula, the unconformity between accreted strata and fore‐arc basin, present along the whole belt, corresponds to a large paleotemperature gap, supporting the occurrence of a major collision in Early Miocene. This tectonic event occurred before the magmatic event that affected the whole belt at ~15 Ma. The associated shortening was accommodated in two opposite modes, either localized on regional‐scale faults such as the Nobeoka Tectonic Line in Kyushu or distributed through the whole belt as in Shikoku. The reappraisal of this collision leads to reinterpret large‐scale seismic refraction profiles of the margins, where the unit underlying the modern accretionary prism is now attributed to an older package of deformed and accreted sedimentary units belonging to the Shimanto belt. When integrated into reconstructions of Philippine Sea Plate motion, the collision corresponds to the oblique collision of a paleo Izu‐Bonin‐Mariana Arc with Japan in Early Miocene.

K‐Ar Dating of Fossil Seismogenic Thrusts in the Shimanto Accretionary Complex, Southwest Japan

“…In the Kodiak accretionary complex, Paleocene igneous intrusives have been attributed to subduction of the Kula‐Farallon (Moore et al, ) or Kula‐Resurrection (Haeussler et al, ) spreading center. The dating results of this study of the Shimanto belt, regardless of the cause, are consistent with Early Miocene‐Middle Miocene out‐of‐sequence faulting in the upper plate (i.e., the margin wedge), and we favor the Raimbourg et al () argument for oblique collision with the Izu‐Bonin arc.…”

“…The Early Miocene is a time of deformation throughout the Shimanto belt, with an early to middle Miocene unconformity in the forearc (e.g., referred to as the Takachiho orogeny, Hibbard & Karig, 1990). This deformation has been attributed to collision with continental crust (Charvet, 2013;Raimbourg et al, 2014;Stein et al, 1994) or with the Izu-Bonin volcanic arc (Raimbourg et al, 2017). Opening of the Japan Sea backarc in the early Miocene may have also impacted the rate and direction of convergence along the Shimanto margin (Regalla et al, 2013).…”

Slab Horizontal Subduction and Slab Tearing Beneath East Asia

“…Latest Oligocene to earliest Miocene may be the time when oblique convergence between PSP and the Eurasian continent initiated, as evidenced by basin inversions in the East China Sea shelf basin (Su et al, ) and shortening at 20 Ma along the Japan margin (Raimbourg et al, ; Figure c). The PSP continued to move northward, and its subduction beneath eastern Asia propagated diachronously along the Ryukyu trench.…”

U–Pb ages of Miocene near-trench granitic rocks of the Southwest Japan arc: implications for magmatism related to hot subduction

“…They also called upon the collision of the palaeo-IBM arc to explain the coastal embayments between capes Ashizurimisaki, Murotomisaki and Shionomisaki. Raimbourg et al (2017) investigated the collision of the palaeo-IBM arc with the SW Japan margin by analysing deformation kinematics and by mapping peak palaeotemperatures through Raman spectra analysis of carbonaceous materials in Cretaceous to Neogene sedimentary rocks of the accretionary Shimanto Supergroup. They claimed that the collision of the palaeo-IBM arc took place in early Miocene time, before near-trench magmatism began in the SW Japan arc.…”