THE OCCURRENCE OF A NEWLY FOUND EOCENE TECTONIC MELANGE IN KARANGSAMBUNG AREA, CENTRAL JAVA

PROCEEDINGS PIT IAGI RIAU 2006
The 35^th IAGI Annual Convention and Exhibition
Pekanbaru – Riau, 21 – 22 November 2006

THE OCCURRENCE OF A NEWLY FOUND EOCENE TECTONIC MELANGE IN KARANGSAMBUNG AREA, CENTRAL JAVA

C. Prasetyadi ¹ , E.R. Suparka ² , A.H. Harsolumakso ² , B. Sapiie ² .

¹ Jurusan Teknik Geologi, FTM-UPNV, Jalan Lingkar Utara, Condongcatur, Jogjakarta.
² Department of Geology, FIKTM- ITB, Jalan Ganesha No.10, Bandung.

ABSTRACT

Based on the 2004-2005 field work in the northern part of Karangsambung area this study presents new evidences for the occurrence of Eocene tectonic melange in the area. Previous studies have mapped the northern area of Karangsambung as part of the Cretaceous Luk Ulo Melange Complex. In this area, especially in the Larangan area, the present study discovered for the first time Middle Eocene Asterocyclina-bearing limestone blocks within the tectonic melange. The occurrence of Middle Eocene limestone blocks indicates the age of tectonic melange rock in this area is no longer of Cretaceous but at least of Late Eocene. If Luk Ulo melange complex indicates the presence of Cretaceous-Paleocene northwestward-plunging subduction zone then it is expected to the north we should find the older part of the melange. The presence of Eocene tectonic melange to the north of the Cretaceous-Paleocene subduction melange does not fit with this model and indicates that deformation phases responsible for both melanges are different. Based on our field work we propose a new model for the tectonic evolution of the area that the shifting of NE-SW trend of Cretaceous subduction to E-W trending in the Oligocene time has occurred due to collision of a micro-continent. It is concluded that the Late Cretaceous to Paleocene subduction occurred in the subduction zone where the Luk Ulo Mélange Complex was formed. The sub-sequence overlying Paleogene (Eocene) series were deposited, following by collision in the Late Eocene – Early Oligocene as indicated by the occurrence of Eocene tectonised rocks prior to the onset of the OAF (Old Andesite Formation) new subduction-related volkanism.