DEFORMATION IN SHEAR ZONE OF THE BUTSUZO TECTONIC LINE UNDER SYN-SEDIMENTARY TO DIAGENETIC CONDITIONS

The Butsuzo Tectonic Line (BTL) is a major fault which separates the Chichibu Belt (lower Paleozoic to lower Cretaceous) and the Shimanto Belt (lower Cretaceous to lower Miocene), Outer Side of Southwest Japan. But brittle to ductile fault shear zone is not always associated is contentious. Fieldwork has been done in the rocky coast of Warabiishi, Anan City, Tokushima Prefecture, where continuous successions from the Chichibu Belt to the Shimanto Belt are exposed. Skills for sedimentary facies analysis as well as structural analysis were called for because major deformation of the activity of the BTL was formed under syn-sedimentary condition as below.

The thickness of the BTL in the study area is about 250m and composed of alternation of sandstone dominant mélange and mudstone dominant mélange. The sandstone dominant mélange is about 50 to 100m thick and composed of lenticular coarse- to medium-grained sandstones (generally 50cm to 5m in thickness) and thin mudstones (generally 5 to 20cm). The mudstone dominant mélange is about 5 to 20m thick and composed of lenses and fragments of sandstones, mudstones, limestones, cherts and basic volcanic rocks in mudstone matrix. The mélange comprise two sets of layers, Y and P. The P-layer (cut by Y-layer) is composed of alternation of mudstones and, lenticular layers of sandstones and varicolored mudstones which are inclined to the Y-layer. The Y-layer is continuous, a few cm to about 1m thick layer composed of inclusions (blocks, lenses, boudines with necking) and varicolored mudstones with thin parallel lamination-like structure. Flow deformed inclusions have σ type asymmetrically swirled structure. Under microscope, the lamination-like varicolored mudstone layers (0.5 to 2mm) have set of continuous Y shears as layer boundaries and P foliations defined by recrystallized clay minerals which inclined to the Y shears. The Y shears have no gauges and dark seams, and just cut P foliations. Riedel shears are also identified. The flow deformation of inclusions and Y shears without gauge suggests that the shear structures might be formed under syn-sedimentary to diagenetic conditions. The pre-lithification structures indicate overall left-lateral displacement.