Brittle normal faulting in the highest-grade Sambagawa metamorphic rocks of central Shikoku, southwest Japan: Indication of the exhumation into the upper crustal level

Abstract

Analysis of fault system in the high- P/T type Sambagawa metamorphic rocks of central Shikoku, southwest Japan, shows that conjugate normal faults pervasively developed in the highest-grade biotite zone (upper structural level) in three study areas (Asemi river, Oriu and Niihama areas). These conjugate normal faults consist of NE–SW to E–W striking and moderately north-dipping (set A), and NNW–SSE striking and moderately east dipping (set B) faults. The fault set A is dominant compared to the fault set B, and hence most of deformation is accommodated by the fault set A, leading to non-coaxial deformation. The sense of shear is inferred to be a top-to-the-WNW to NNW, based on the orientations of striation or quartz slickenfibre and dominant north-side down normal displacement. These transport direction by normal faulting is significantly different from that at D 1 penetrative ductile flow (i.e. top-to-the-W to WNW). It has also been found that these conjugate normal faults are openly folded during the D 3 phase about the axes trending NW–SE to E–W and plunging west at low-angles or horizontally, indicating that normal faulting occurred at the D 2 phase. D 2 normal faults, along which actinolite breccia derived from serpentinite by metasomatism sometimes occurs, perhaps formed under subgreenschist conditions (ca. 250 °C) in relation to the final exhumation of Sambagawa metamorphic rocks into the upper crustal level. The pervasive development of D 2 normal faults in the upper structural level suggests that the final exhumation of Sambagawa metamorphic rocks could be caused by “distributed extension and normal faulting (removal of overburden)” in the upper crust.