Seismic investigations of subduction and intra-arc rifting at the Hikurangi margin, New Zealand




Gase, Andrew C.

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Subduction zones are dynamic systems that control the global distribution of large earthquakes and volcanism. Many interrelated factors can control tectonic, seismic, and magmatic processes within subduction zones, including mechanisms that vary stress, thermal regime, volatile supply, as well as inherited features within the lithosphere, but the relative importance of these factors are debated. North Island New Zealand, where the Pacific Plate subducts beneath the Australian Plate, is renowned for its unique patterns of seismicity and plate coupling in the forearc, the Hikurangi margin, and its magmatically productive intra-arc rift, the Taupo Volcanic Zone. In this dissertation I present three studies that use newly acquired controlled-source seismic data to evaluate (1) crustal and sedimentary controls on seismic behavior in the Hikurangi margin forearc, and (2) interplays between magmatism and crustal deformation in the offshore Taupo Volcanic Zone. In the first study, I explore the crustal structure of the northern Hikurangi margin, which is world renowned for its low seismic coupling, frequent shallow slow slip events, and strong ground-motion amplification during large earthquakes. I show that sharp along-strike variations in frontal accretion indicate variable sediment supply and past subduction of seamounts. Low velocities in the overthrusting plate indicate the presence of compliant materials that likely contribute to tsunamigenesis and enhanced ground motion during earthquakes. In a second study, I compare the structure of the megathrust fault across the interseismic coupling transition between the central and southern Hikurangi margin and reveal a clear correlation between sediment subduction and slip behavior. In the northern and central unlocked, slow slipping segments, the megathrust forms within pelagic carbonates and volcanic sediments. In contrast, the southern locked megathrust is localized to pelagic carbonates and is insulated from the effects of volcanics by ~0.5-1 km of subducting clastic sediment. I propose that slip behavior and coupling is controlled by the lithology and spatial distribution of frictional asperities along the megathrust. Finally, I determine the crustal structure of the offshore Taupo Volcanic Zone and demonstrate that crustal extension and recent magmatic activity are collocated. Deep-penetrating crustal normal faults overlie a ~40-kilometer-wide zone of sill-complexes and heterogeneous seismic velocities in the upper and middle crust. I propose that magmatic intrusions are localized by more permeable fractured crust and contribute to thermal weakening which facilitates rifting.


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