Neotectonics of the Thoen fault system, Lampang basin, Northern Thailand

Abstract

Neotectonics of Thoen fault zone in the northern Thailand is far from understood, mainly because of the poor tools geologists have in assessing neotectonic structures and determining active faults in basement rocks of the Cenozoic basin. The major faults along the eastern edge of the Cenozoic basin were previously estimated from imagery remote-sensing interpretation on regional scale and no detail of remote-sensing and field based data are investigated. The behavior of the tectonic stress field in basement rocks and its relation to the neotectonics in Cenozoic basins are within poorly understood. The study area is located within the N-NE striking Thoen fault zone, with a total length of about 120 km cross the Mae Moh, Lampang, and Thoen basins. The three main methods of study, remote-sensing study, petrographic study, and subsurface study, show good results for determining the neotectonic structure in the study area. Detailed result from lineament analysis had lead to the good field evidence of morphotectonic and Quaternary faulting Rock sampling for petrographic analysis, and six exploratory locations for ground penetrating radar (GPR) were performed subsequently using the results of field surveys. The overall results lead to the reconstruction of evolution model and the Thoen fault system is defied as Neotectonic fault zone which is the youngest structure formed since 40 Ma age after Indian-Eurasian collision. The NE-SW oblique-sinistral strike-slip movement is agreed with the dominantly current tectonic regime. The very shallow subsurface reverse faulting, with dip to SE about 12-20 degrees apparently were clearly detected from GPR profiles in Cenozoic basin. This suggests that tectonic process have occurred in the current tectonic regime, so its movement is likely to be the "active tectonics" definition. The revised method for active tectonic study is also proposed.