Mechanisms of lithospheric failure during late continental rifting and early subduction

Two fundamental components of plate tectonics are the separation of continents, leading to new ocean basins, and the initiation of subduction zones, which facilitate recycling of the Earth's outer shell into its interior. In order for continental rifting and subduction initiation to succeed, tectonic driving forces must overcome resisting forces and strength of the lithosphere. If achieved, the lithosphere undergoes failure and a new plate boundary is established, wherein subsequent strain is localized along a narrow weak zone, such as a subduction zone megathrust or seafloor spreading center. Though these processes are conceptually straightforward, many aspects remain elusive. In particular, intact lithospheric strength is thought to be far greater than available tectonic forces, yet observationally continental breakup and subduction initiation occur frequently throughout Earth's history. The goal of this dissertation is to further investigate this force paradox by exploring the weakening mechanisms that assist lithospheric failure during late continental rifting and early subduction. Active-source seismic data are used to image geologic processes and the tectonic evolution along two study areas - the Eastern North American Margin and the Puysegur Margin, New Zealand. Along the Eastern North American Margin, I show that new mafic crust was emplaced above a thinning subcontinental mantle lithosphere that resisted breakup despite abundant magmatism. I propose a new model in which continental crust separated before the lithosphere and complete breakup was not achieved for ~25 Myrs after the arrival of melts. I then image mantle dynamics near the lithosphere-asthenosphere boundary during final stages of rifting and show that rupture was enabled by highly organized crystallographic textures that focused melt and deformation into a narrow weak zone. At the Puysegur Margin, I argue that subduction initiation was aided by previous phases of continental rifting and strike-slip. Rifting stretched continental crust of Zealandia and later dextral strike-slip translated thin and dense oceanic crust from farther south and juxtaposed it with thick continental crust at a collisional restraining bend. Ideal conditions ensued, where buoyancy contrasts and pre-existing fault zones weakened the lithosphere and facilitated subduction nucleation. Since initial underthrusting, subduction initiation became more efficient as the trench propagated southward over time. I conclude with a novel 4D model where subduction initiation is resisted at the site of nucleation but followed by mechanically easier and faster initiation and lateral propagation as the plate boundary develops along-strike. Inherited lithospheric heterogeneities and weak zones are the dominant mechanism allowing the plate tectonic cycle to persist on Earth.