Thin Viscous Sheet Models for Distributed Deformation of Oceanic Lithosphere Accommodating Motion Between the India, Capricorn, and Australia Plates

While it has been long recognized that plate boundaries through continents were typically diffuse, it has taken longer to recognize the extent of diffuse plate boundaries in the oceans. The strain rates in diffuse oceanic plate boundaries are typically lower than in diffuse continental boundaries, but about the same fraction of oceanic lithosphere is diffuse boundary as is found in continental lithosphere. Many fundamental questions remain unanswered or poorly constrained. E.g., What controls the length-scale of this deformation? What is the rheology of deforming oceanic lithosphere? What controls the sense and style of deformation? Displacement rates range up to ≈15 mm a^{-1} and strain rates up to 10^{-16} s^{-1}. Diffuse oceanic plate boundaries are much wider than traditionally defined narrow plate boundaries such as mid-ocean ridges for which the main boundary zone is 1 to 2 km wide. In contrast the diffuse boundary between the India and Australia plates seaward of the Java-Sumatra trench is thousands of km wide. The inferred strain rates are one to three orders of magnitude greater than the average oceanic intraplate strain rate inferred from seismic moment release or thermal contraction in old oceanic lithosphere, but many orders of magnitude lower than in traditional narrow plate boundaries. In this presentation, we review some key observations of diffuse oceanic plate boundaries and present an investigation of the rheology and dynamics of the India-Capricorn-Australia plate boundaries mainly through the application and analysis of thin viscous sheet models.