Understanding Arc-Continent Collision in the Banda Arc Through 3-D Seismic Imaging

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. From west to east along the Sunda-Banda arc, the subducting Indo-Australian plate changes from the Cretaceous to Jurassic age Indian Ocean lithosphere to continental lithosphere of Australian margin as it collides with the Banda arc. This young collision effectively captures spatial transition between subduction zone to arc-continent collision, which can be alternatively viewed as the temporal transition of these processes along-strike. Detailed knowledge of seismic structure is key to understanding this complex tectonic transition. A new Banda Arc Seismic Experiment composed of 30 broadband stations was carried out from March 2014 to August 2019 (~5 years) in the Timor-Leste and Nusa Tenggara Timor region of Eastern Indonesia. In this thesis, I analyzed this dataset with multiple techniques including receiver function, auto-correlation, ambient noise tomography, as well as teleseismic surface wave tomography, aiming to image the first comprehensive and high-resolution 3-D crust and upper mantle seismic structures at different scales. An up-to-date seismic catalog that is foundational for structural studies was also built. I successfully imaged the subducted Australian continental margin at lithospheric depths, with pronounced along-strike structural variations at different scales. I suggested it may reflect the diachronous (progressive) collision as a result of oblique plate convergence or the inherent structural heterogeneities of the incoming and colliding Australian (lower) plate, or both. The present-day crustal structure may also be complicated by exotic terranes or microplates formed during the Jurassic breakup of eastern Gondwana. Tectonic fabrics related to orogenic, strike-slip, and volcanic structures from a variety of crustal depths are imaged across the entire collisional zone. The distribution of their strikes is interpreted to be associated with orogenesis, variable magmatic systems, strike-slip motion, and strain partitioning in response to the arc-continent collision and plate convergence. The new catalog describes a complex pattern of crustal events and abundant deep slap seismicity. Lastly, I analyzed the observed crustal fabrics, mantle anisotropy, isotropic structures in the crust and upper mantle as well as the seismicity together with the topography and existing geochemistry and geology observations. Altogether, these new seismic observations make a strong case links the surface geology with subsurface crust and mantle structures, unraveling an enigmatic structural and compositional boundary along the Banda volcanic arc.