Subduction zone segmentation along the Sunda Margin, Indonesia

The Sunda margin marks the eastern termination of the Indian Ocean along the Indonesian archipelago, where the subduction of the Indo-Australian plate below Eurasia is taking place. The convergence direction changes from orthogonal in the eastern part of the margin to a more oblique convergence towards the north-west offshore Sumatra. Along the margin, a wide range of subduction scenarios is observed, which makes this margin a perfect natural laboratory to study the role of various physical and geological parameters on subduction dynamics. This work builds on geophysical and tectonic interpretations of data acquired along eight seismic/gravity and several MCS marine profiles, distributed along the Sunda margin. The data include wide-angle seismic records, multi-channel seismic data, and gravity data. Data interpretation and modeling include a joint reflection/refraction seismic tomography used to constrain geophysical models across the subduction complex at several locations, further constrained by geological information from the multichannel seismic data. Additionally, other geophysical information including gravity modeling and bathymetry data is employed to refine geodynamic models based on seismic interpretations. Excellent data quality allows detailed structural models of the subduction complex to be constrained in several locations along the Sunda margin. The geophysical models obtained in this study show significant variations of the crustal and upper mantle structure of the subduction complex along-strike and across-strike of the margin. The study revealed an increased thickness of the crystalline crust in the Savu Sea, attributed to the approach of the Australian shelf to the trench. Offshore Lombok Island, the oceanic crust thickness is found to be 7 km, and heavily fractured by normal faults. The crustal structure of the Roo Rise oceanic plateau was modeled for the first time, revealing the crustal thickness of 15 km. Its subduction is causing inhomogeneous deformation of the forearc and complex evolution of the entire subduction processes. In addition, large variability in the amount of sediment on the incoming plate is found in adjacent sectors of the margin, ranging from ~3 km in the Savu Sea to almost absence in the Lombok and East Java areas. Offshore north Sumatra, comparison of the crustal-scale profiles located on different sides of the segment boundary between 2004 and 2005 earthquakes rupture areas, showed variability of the structure of the oceanic plate and the amount of sediment present at the trench, resulting in the principally different sizes of the accretionary prisms and the width of the seismogenic zone. A comparison of the subduction complex along the adjacent profiles shows that changes in the structure of the incoming oceanic plate result in drastic changes in the structure of the subduction complex and its evolution. New crustal-scale geophysical models enable analysis of the geodynamic effects caused by the transition from oceanic subduction to continental shelf collision, the presence of anomalous relief on the oceanic plate, variations in the morphology of the incoming oceanic plate, and the presence of segment boundaries, and thus provide a solid basis for geodynamic interpretations of subduction processes and the associated geohazards.