Characteristics and Mechanism of Back-Arc Fold-Thrust-Belt an Implication to Earthquakes Hazard
Benyamin Sapiie, Meli Hadiana, Terry Alfa Furqan, Rayhan Tirto Nugroho

Prodi Teknik Geologi, Institut Teknologi Bandung

Abstract

Fold-thrust belt (FTB) is one of the most complex deformations of sedimentary rocks. Back-arc FTB is one of the kinds observed in active subduction tectonic settings. This FTB raises a question concerning their mechanism and geologic controlling factors mainly related to earthquakes hazard. The knowledge of structural development, such as FTB resulting from rock deformation, is limited due to poorly known in situ rock properties. Investigating rock deformation using a sandbox modeling approach can solve these problems because it can relate and compare the initial undeformed stages with their later deformed stages. This comparison is critical in understanding the structural development or evolution resulting from the specific setting of rock deformation, particularly in the sedimentary basin. Sandbox modeling has been used widely in structural geology to understand complex deformation patterns such as fold-thrust-belt and strike-slip fault systems. The experiment uses standard custom-built sandbox apparatus in the ITB sandbox-modeling laboratory. The device used in this experiment is an open-topped glass box with fixed external walls and one or two moveable internal walls pushed by screw jacks driven at a constant velocity by a stepper motor. In this study, sand grains are natural dry quartz sand with a Navier-Coulomb rheology and an angle of internal friction of about 30o, similar to many sedimentary rocks. Particle Image Velocimetry (PIV) was used to evaluate strain distributions and fault sequences. The results of the PIV analysis show that there is evidence of segmentation in the model. This segmentation marked the difference in speed between fault sequences. The fault pattern, geometry, and style produced by the model are very similar to the pattern mapped like the back-arc FTB (for example, the Baribis system) in nature. Therefore, it can be concluded that the fault mechanism in FTB is highly influenced by lithology variation and pre-existing basement faults.

Keywords: Back-Arc Fold-Thrust Belt, Sandbox modeling, deformation, earthquakes hazard

Topic: Solid Earth Sciences