Subduction of Seamounts and Ridges along the Java Margin, Indonesia: Impacts on Structural Geology and Seismic Activity

The Java-Lesser Sunda margin exhibits different topography of subducting oceanic basement relief and diverse upper plate tectonic processes, ranging from neutral characteristics offshore Lombok and Sumbawa to erosional features offshore Central Java to Bali, distinct from its accretionary counterpart off Sumatra. Despite this classification, a comprehensive understanding of how the subduction of oceanic basement relief influences the plate boundary and upper plate structure across the transition from neutral to erosional remains elusive. In our investigation, we illuminate the tectonic parameters governing the margin's classification by integrating multi-channel reflection seismic images obtained through a grid-based P-wave velocity inversion and high-resolution multibeam bathymetric maps. Our dataset reveals the nuanced modifications to seafloor morphology, upper plate structure, and décollement position brought about by various scales of subducting topography. Large-scale subducting features prompt a landward shift of the deformation front, leading to a shortened accretionary wedge and heightened seafloor incline at the relief's trailing edge. Conversely, small-scale subducting ridges predominantly impact the frontal prism, causing over-steepening at the trench and localized slope failures. Deformation of the accretionary wedge ahead of subducting relief is characterized by intensified compression and reduced seafloor slope, seemingly independent of the relief's size. Ridge and seamount subduction induce frontal erosion and basal erosion offshore Lombok and Bali, respectively. Our P-wave velocity models reveal a notably lower rigidity of the upper plate's base along the eastern Sunda margin compared to the global trend. This lower rigidity is a crucial factor favoring the occurrence of tsunami earthquakes on the Java margin. In conclusion, our study provides a comprehensive analysis of the complex interplay between subducting oceanic relief and tectonic processes, shedding light on the factors that dictate the margin's transition from neutral to erosional characteristics and the associated seismic implications.