3D PALINSPATIC RECONSTRUCTIONS OF RAJAMANDALA CARBONATE COMPLEX AS IMPLICATION OF PALEOGEOGRAPHY IN THE WESTERN JAVA, INDONESIA

摘要

Rajamandala Carbonate Complex, known as Rajamandala Formation, which outcrops in the Padalarang area, West Java, is one of the best outcrops for representing an Oligocene-Miocene limestone formation on Java Island. The main objective of this study is to present results of 3D palinspastic reconstructions using balanced cross-section techniques using field geological cross-sections. These were used to interpret the paleogeography and for verifying tectonic evolution of the region. The result of structural mapping in the RajamandalaCarbonate Complexindicates that all major lithologic units have been folded and faulted by a north-verging thrust-fault system. Thefaults strike NE-SW and are south dipping with an average dip of 40-70 degrees. Folds are found in the hanging wallindicating they are associated with a thrust fault. Nine NW-SE geologic cross-sections were constructed in this study. Balanced cross-sections were derived from five cross-sections which indicate that the study area underwent shortening between 48-52%. 3D palinspastic reconstruction models suggest the style of deformation was via thin-skinned imbricate duplex fold-thrust-belt systems with the detachment levels calculated at a depth of 700 to 1300 meters below sea level. The amount of shortening for each location is not similar which suggested an asymmetrical deformation where some blind thrusts must be present at depth. Thrust faults are cross-cut in several areas by NW-SE dextral strike-slip faults. These strike-slip faults are interpreted as tear faults associated with an asymmetrical deformation during formation of thrust-fault system. Our model, supported by fieldwork, indicates NE-SW trending embayment was present in West Java during Oligocene time where Rajamandala Limestone was developed and distributed as a platform carbonate complex. This embayment may be controlled by a major discontinuity known as Cimandiri Fault representing plate boundary during early Tertiary. The youngest deformation of the Plio-Pleistocene occurs parallel to this pre-existing structure suggesting basement involved deformation. The modeling results indicate the deformation style within the Rajamandala Formation is most likely via thin-skinned fold-thrust belts.