Schematic formation of boudine granite and microfold phyllite of Gilas River: Implications for Triassic to Tertiary tectonics of Garba Hill, South Ogan Komering Ulu Regency, South Sumatra Province

Gilas River is one of the areas that shows interesting geological structures due to the meeting between West Sumatra and the Woyla Block. This boudine and microfold formation scheme aims to reveal the tectonics of Sumatra Island during the Late Cretaceous to Tertiary period, especially those found in the Gilas River. The methods used are DEMNas (National Digital Elevation Model) analysis, outcrop observation, and deformation element data analysis. The data obtained is then used to reconstruct structural architecture models, force-related deformation mechanisms, and the tectonic evolution of the area. The Gilas Strip experienced several tectonic events based on the results of research at the observation site. Based on the results of the analysis, it is known that the first structure is a foliation in the phyllite rock which is oriented ESE-WNW due to the first deformation in the Triassic period. The second deformation occurred in the Late Cretaceous period which produced the NNE-SSW trending Dextral Milling Fault (S2). The formation of micro folds with the fold axis having a position of N118°E/80 found at observation location IWD 4 and boudine structure with ENE-WSW direction at observation location IWD 5 is the result of the second deformation (D2). The Gilas Right Horizontal Fault produces an antithetic fault in the form of a left horizontal fault with a relative direction of E-W (S2′). The last period is the formation of a conjugate shear fault at the observation location IWD 3 due to Neogene tectonic deformation (D3), with a relative N-S fault direction.