Internal and permeability structures of Pre-Cretaceous nappe boundaries in the Inner Zone of Southwest Japan

Abstract

We determined the internal and permeability structures of the nappe boundary between the Tamba and the Ultra-Tamba belt in the Inner Zone of Southwest Japan. Protolith of hanging wall is Permian sandstone and that of footwall is Jurassic pelitic breccia. These are stuck tightly with cataclasite zone of 3 m in width. Permeability data and structural observations of the fault zone suggest the following; (1) cataclasite is distributed in the fault zone, but most deformation is localized in 0.5 to 3 cm width foliated cataclasite zones in the hanging wall, (2) experiments at effective pressures, P_e, of 40 to 100 MPa show that permeability of the hanging wall is low (10^-20 m² at P_e > 80 MPa), while it is higher by 1-3 orders of magnitude in the footwall and the fault zone. The permeability structure implies that high pore fluid pressure could be sustained within the fault zone, which could have promoted the thrust movement.