Integrating seismological and tectonic studies to constrain lithospheric evolution at complex plate boundaries

The relative motion of tectonic plates across their boundaries generates deformation in the surrounding lithosphere. How this deformation is expressed reflects both present-day plate configurations and how plate boundaries evolve. To understand the behavior of plate boundaries, we must study how they have developed. The advances made in seismology over the past fifty years---both in observation and application---provide tools ideal for such analysis. Here, I use these tools to investigate the tectonic evolution of complex plate boundaries. I focus on two areas that have experienced geologically recent plate tectonic variations---the Mendocino triple junction in northern California, and the Australia:Pacific plate boundary south of New Zealand. In northern California, the northward migration of the Mendocino triple junction over the past ∼10Ma has driven a synchronous pattern of thickening and thinning of North American crust. In studying this deformation I find major thinning is localized to a narrow region of crust in the Redwood Valley area. This thinning is accompanied by a steeply dipping Moho (>15°), and by high Poisson's Ratio's in the lower crust, characteristic of layers of melt. These melts may link to shallow (∼10km) dike injections that drive a migrating sequence of seismicity in the shallow crust near Lake Pillsbury. The limited data sets available led me to develop new tools in receiver function and crustal velocity ratio analyses that significantly improve our ability to resolve spatial changes in crustal properties. The application of these techniques forms a second major aspect of this thesis. Finally, I analyze the deformation of Australian lithosphere adjacent to the plate boundary south of New Zealand. I perform earthquake relocations on the distribution of intra- and inter-plate seismicity, and combined with plate reconstructions since the late Oligocene show that deformation occurs over a ∼150km wide area west of the current plate boundary. This may be related to resistance to subduction of the Australian Plate at the Puysegur Trench further north. Persistence of this stress transfer may lead to a southward jump in the subduction zone and a capturing of this lithospheric block by the Pacific Plate.