The Flores Thrust and Its Interplay with Arc Volcanism

The 2018 Lombok earthquake demonstrated the hazard that the Flores Thrust possess and highlighted our lack of knowledge on this crustal-scale back thrust. The Flores Thrust extends from west to east, located at the back of the arc in the subduction zone of the Lesser Sunda Islands, Indonesia, as a result of an extensive crustal deformation of the Sunda upper plate by the northward motion of the subducted Indo-Australian plate. Here we construct a geometry model of the Flores Thrust by fitting the best surface on the relocated earthquake catalogues using simulated annealing. The thrust has a flat and a ramp component, where the flat is generally constant, inclined ~5˚ southward. The ramp of the thrust is dipping southward with 38˚ average dip, where the dip is increasing 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˚). Based on our analysis of volcano distribution, the Flores Thrust is constrained by arc volcano distribution; bounded by volcanoes at the eastern and the western tips while the deeper part of the thrust is terminated by the along-arc distribution of volcanoes. The clustered distribution of earthquakes and the occurrence of strike-slip focal mechanisms between volcanoes indicate the possibility of the segmented geometry of the thrust, perhaps due to strain localisation by the thermal disturbance from the volcanoes. The Flores Thrust might also affects the arc by migrating the active volcanism northward in Sumbawa Island, leaving extinct edifices in its south while forming the highly potassic rear-arc volcanoes, Tambora and Sangeang Api, in the north. The relatively constant crustal thickness, SiO2, U, and Th contents along the arc while Sr and Nd peaks at Tambora and Sangeang Api imply the higher crustal assimilation in these two volcanoes, possibly induced by the propagation of the Flores Thrust as it also migrates the volcanism. It also indicates the possibility of the initial propagation location of the thrust in this region. 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 geophysical hazards in the region.