[S-CG65_2AM1] Stress and Crustal Dynamics

★ [SCG65-01] Reconstraction of paleostress states in the Walanae fault zone in South Sulawesi using the multiple inverse method with

*Osamu NISHIKAWA¹ (1.Akita University)

Keywords:multiple inverse method, calcite twin, Walanae fault zone

Paleostress analysis using the multiple inverse method with calcite twin data was performed in the East Walanae fault (EWF) zone in South Sulawesi, Indonesia. The geomorphic trace of the EWF can be recognised as a distinct line between the Bone Mountains and the Walanae Depression, around which an intensive deformation zone characterised by various scales of faults and folds are developed. Carbonate rocks with numerous calcite twins and mesoscale faults are ubiquitous around the EWF trace. Therefore EWF zone is a useful location for testing the inclusion of calcite twin data in the multiple inverse technique to determine paleostress states. One to three poles of differently oriented twin lamellae and c-axis orientation were measured for each grain from three mutually perpendicular thin sections for 11 samples using a U-Stage optical microscope. The data set for multiple inverse method consists of the attitude of the e-plane, gliding direction and sense of shear of e-twinning. We prepared data files not only for twinned e-planes but also for the remaining untwinned e-planes in a grain with one or two twin sets. We incorporated the untwinned e-plane data for determining stress states with the multiple inverse method using calcite twins. In the analysis, the identified stress states by twinned e-plane data were tested calculating misfit angle β, the angle on the untwinned e-plane between the calculated maximum shear stress direction for every identified stress state and the observed potential gliding direction. It is possible to say that the sampled rocks had never experienced stress states to activate any of the untwinned e-planes. Therefore, if most untwinned e-plane data (95% or more in this study) are incompatible( β > 30 degree ) with the stress state identified from twinned plane data, then the stress state is viable for both the twinned and untwinned e-planes.The analysis using calcite twin yielded reliable paleostress states similar and consistent with those from fault-slip data throughout the study area. Dominant and common stress states are characterized by NE-SW-to-E-W-trending σ1 and vertically to moderately-south-plunging σ3 with generally small values of stress ratio φ. These stress states were most likely caused by collision of eastern Sulawesi with the Australian fragments since the Pliocene, and they could have activated the EWF as a reverse fault with a dextral shear component, accounting for the contraction deformation structures and landforms along the trace of the fault. Calcite twin and mesoscale faults were activated predominantly during the fold tightening stage and subordinately before folding.