THICKENING OF TECTONIC MéLANGES DUE TO CLAY-MINERAL TRANSITION IN 2-4 KM DEPTH IN SUBDUCTION ZONE: HOTA ACCRETIONARY COMPLEX, CENTRAL JAPAN

The Initiation of tectonic mélange formation and related-clay mineral variation in aseismic-seismic transition zone along subduction-related thrust were examined by means of investigating onland accretionary complex buried only 2-4 km. The lower to middle Miocene Hota accretionary complex, central Japan, is a unique example of onland accretionary complex, representing initiation of tectonic mélange formation and relation between deformation and physical/chemical properties variation of sediments just prior to entering the seismogenic realm. Two kinds of subduction- related deformations: one is the phacoidal deformation (D1) characterized by rhombus-shaped fragmented mudstone with thin zone of clay-mineral preferred orientation in the outer rims whereas random fabric inside, similar to the primary décollement-zone deformation recovered by ocean drilling toe of modern accretionary prism; the other is deformation characterized by block-in-matrix style deformation (D2) with asymmetric S-C structure and intense clay-minerals preferred orientation, similar to the deformation of tectonic mélange in matured ancient décollement zone, are preserved in the Hota accretionary complex. Considerable-smectite reduction and illite increment were clearly identified inside the D2. Disappearance of weak smectite that has small frictional coefficient, and increment of electric cohesion of sediment associated with porosity reduction and clay mineral preferred orientations during D2 formation lead the shear zone strengthening. Such strain hardening associated with dynamic clay-mineral transition and particle orientation would be the primary mechanism in mélange zone thickening and fundamental mechanical transition just prior to entering the seismogenic zone.