Crustal structure and its tectonic implications to the Banda Arc-Australian continent collision revealed by teleseismic receiver functions

Abstract:

The active arc-continent collision in the Banda Arc is a result of convergence of the Indo-Australian plate with the Eurasian plate in SE Asia. There, the subduction of the Cretaceous to Jurassic age Indian Ocean lithosphere along the Sunda Arc transitions eastward to the recent collision of the Australian continental lithosphere with the active volcanic arc. Such complicated convergent tectonics leads to pronounced structural heterogeneity and complexity, where both anisotropy and dipping layers together with strong isotropic seismic velocity jumps are expected. Therefore, a detailed understanding of the deep structure in the Banda Arc is key to unraveling the dynamic processes that occur throughout the convergence. In this study, we analyze ~4 years’ worth of radial and tangential receiver functions observed at 30 newly installed broadband seismic stations in eastern Indonesia and Timor-Leste to constrain both isotropic and anisotropic crustal structure. Preliminary results show significant lateral variability in the crustal structure both along-arc and along the orogenic strike. In particular, our initial observations at stations near the active volcanoes on Flores show evidence for mid-to-lower crustal melts (as a low-velocity zone), analogous to findings in other volcanic arcs worldwide. We also observe backazimuth dependent receiver function arrivals throughout the region, which are indicative to the presence of directional structural features (dipping layers or anisotropy with plunging symmetric axis) from varied crustal depths. The strikes of these structures are roughly parallel to the orogen across the outer arc islands of Timor and Sumba and present a complex orientation pattern on the inner arc island of Flores, in association with the collisional processes and island arc volcanism. Our study is the first to probe the detailed crustal structure in the Banda Arc with passive-source seismology and underlines the importance of applying 3-D seismic imaging in complex tectonic systems.