An assessment of low-temperature, ductile lithospheric deformation using existing broadband seismic data from around South Island, New Zealand

Plate tectonics involves rigid lithospheric plates moving over an asthenosphere which can flow on geologic time scales. However, there is new evidence that the ‘rigid’ lithosphere might be able to deform. This project will study if ductile behavior – when material flows instead of breaking – is present at the Alpine Fault in South Island, New Zealand. Results will have implications for global tectonic processes. Broader impacts include support for an early career scientist and dissemination of software related to the project. The main goal of the proposed research is to assess whether shear stress leads to low-temperature ductile deformation in the lithosphere. Seismic anisotropy, a proxy for viscous strain, will be used to test whether ductile deformation is occurring at low temperatures in the lithospheric mantle around the Alpine Fault in South Island, New Zealand. While previous studies have shown that strain associated with relative motion across the Alpine Fault is accommodated in a zone up to 200 km wide, prior anisotropy measurements from shear wave splitting and earthquake Pn have not provided sufficient spatial resolution to determine whether deformation is occurring in the lithosphere or the asthenosphere away from the plate boundary. An existing ocean bottom seismic dataset will be used to calculate receiver functions, applying recently developed techniques for removing sediment and water column reverberations, and harmonic decomposition will be used to measure azimuthal anisotropy in the lithospheric mantle on both sides of South Island. This will resolve lateral and vertical variations in seismic anisotropy around the Alpine Fault and determine where ductile deformation is occurring in the lithosphere-asthenosphere system. This project is supported by the Marine Geology and Geophysics program in the Division of Ocean Sciences and the Geophysics program in the Division of Earth Sciences. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.