Paleocene large-scale normal faulting along the Median Tectonic Line, western Shikoku, Japan

摘要

The Median Tectonic Line (MTL) in southwest Japan, a major east-west-trending are-parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. The juxtaposition by faulting occurred after the deposition of the Izumi Group. Based on detailed fieldwork and previous studies, the authors reconstruct the kinematic history along the MTL during the Paleogene period, which has not been fully understood before. It is noted that although the strata of the Izumi Group along the MTL dip gently, east-west-trending north-vergent folds with the wavelength of similar to 300 m commonly develop up to 2 km north from the MTL. Along the MTL, a disturbed zone of the Izumi Group up to 400 m thick, defined by the development of boudinage structures with the transverse boudin axis dipping nearly parallel to the MTL, occurs. Furthermore, east-west-trending north-vergent folds with the wavelength of 1-5 m develop within the distance up to 60 m from the MTL. The disturbed zone with the map-scale north-vergent folds along the MTL, strongly suggests that they formed due to normal faulting with a top-to-the-north sense along the MTL. Considering that the normal faulting is associated with the final exhumation of the Sambagawa metamorphic rocks, and its juxtaposition against the Izumi Group at depth, this perhaps occurred before the denudation of the Sambagawa metamorphic rocks indicated by the deposition of the Lower Eocene Hiwada-toge Formation. Dynamic equilibrium between crustal thickening at depth (underplating) and extension at shallow level is a plausible explanation for the normal faulting because the are-normal extension suggests gravity as the driving force.