The Geometry and Thermal Fault Models of Flores Thrust

Abstract

The 2018 Lombok earthquake demonstrated the hazard that the Flores Thrust possesses and highlighted our lack of knowledge on this crustal scale backarc thrust. The Flores Thrust extends from west to east, located at the backarc of the Lesser Sunda Islands, Indonesia. Here we construct the geometry model of the Flores Thrust by fitting the best surface on the relocated and filtered intraplate seismicity using simulated annealing and interpolating using the FastRBFTM algorithm. The thrust has a flat and a ramp component where the flat is generally constant, inclined ~5° southward, but limited between eastern Bali and western Sumbawa due to the sediment-filled North Bali-Lombok Basin. The ramp of the thrust is dipping southward with a 38° average dip, where the dip increases eastward from Bali (27°-37°) to Lombok (28°-38°), Sumbawa (30°-50°), and western Flores (40°-58°), but then shallower in central Flores (29°-39°). The segmented geometry of the Flores Thrust with varying dips from west to east might be related to the complex geological history and the heterogeneity of both the upper and lower plates in the Lesser Sunda region. Based on our analysis of the volcano distribution, the seismogenic zone of Flores Thrust is constrained by arc volcano distribution; bounded by volcanoes at the western tip (Raung, Ijen, and Baluran in Java) and the eastern tip (Lereboleng and Lewotobi in Flores) while the deeper part is terminated by the along-arc distribution of volcanoes or where the temperature exceeds the brittle-ductile transition zone (>450°C). The proximity between the Flores Thrust and the volcanism might also suggest its interplay during the thrust development. The presence of high-K backarc volcanoes, Tambora and Sangeang Api, to the north of the low-K basaltic-andesitic dominated volcanoes in Sumbawa (e.g., Sangenges and Sorumundi) suggest a possible northward arc migration, closer to the thrust, and a complex interaction between arc volcanism and thrust development. Therefore, the complexity of the Flores Thrust geometry and its interplay with the volcanism should be investigated further, to mitigate the greater effects of any geological hazards in the region.