Outcrop to petrography scale and new age constraint of the Jatisamit Mélange in the Luk Ulo area, Central Java, Indonesia

To resolve the formation of the Luk Ulo Mélange in the Karangsambung area of Central Java, the matrix-dominated Jatisamit Mélange, which is a member of the Luk Ulo Mélange, was examined. Outcrop and petrographic observations of sandstone boudins and their shale matrix from the Jatisamit Mélange were used to assess deformational mechanisms and formation. We suggest the Jatisamit Mélange underwent three deformation phases: (1) formation of a web structure in the sandstone boudins through a cataclasis mechanism; (2) formation of boudins and cleavage through a ductile deformation; and (3) formation of open fractures and Riedel shears through pressure increasing. These deformations occurred at relatively low to medium temperatures during progressively increasing differential stress. Using illite crystallinity, the matrix of the Jatisamit Mélange has a maximum metamorphic temperature of 150–200 °C (diagenesis zone) at a 4–5 km depth. In addition, calcareous nannofossil assemblages of the Jatisamit Mélange and other related rock units in Karangsambung suggest a depositional matrix age of middle Eocene–early Oligocene. Therefore, the mixing of the Luk Ulo Mélange occurred not earlier than the middle Eocene–early Oligocene at a shallow structural level in the suture zone during the collision of the southeastern margin of Sundaland with the Gondwana micro-continent