Investigating trench sediment consolidation and upper plate structures and their links to seismic behavior using active-source 2D seismic data in south-central Chile and Hikurangi




Olsen, Kelly Marie

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Subduction zones are convergent margins where one tectonic plate slides beneath another and is recycled into the mantle. Pelagic, hemi-pelagic, and terrigenous sediment deposited onto the subducting plate is off-scraped onto the overriding plate to form an accretionary prism or subduct beneath the overriding plate on top of the subducting plate, where the sediment is either accreted to the base of the upper plate or subducted to at least volcanic depths, or a combination of these processes can occur. Here, I use 2D active-source marine seismic data to create pre-stack depth migrated images and p-wave velocity models to investigate properties of accreted and subducted sediment and examine accretionary wedge and shallow upper plate structures on two thick-sedimented subduction zone margins, the south-central Chile and Hikurangi margin. These two subduction zones have very different seismic behavior; the south-central Chile margin is the location of the largest and sixth largest instrumentally recorded earthquakes, and has a long history of great earthquakes, while the Hikurangi margin has no evidence of very large ruptures, but both deep and shallow slow slip events are observed along this margin. I first examine the south-central Chile margin using 13 seismic transects across the trench and frontal wedge in south-central Chile to investigate along-strike variations in trench sediment properties to understand the partitioning of trench sediment into accreted vs subducted sediment, and link the properties of the subducting sediment to large megathrust earthquakes that have occurred along this margin. I then analyze upper plate structures along the 13 transects, such as thrust and normal faults imaged in the slope sediment, and discuss the relationship between the mechanisms that formed the faults, the stability of the wedge, and how wedge stability may vary throughout the seismic cycle. I then focus on the Hikurangi margin, where the Hikurangi Plateau, part of the Pacific Plate, subducts beneath the Australian Plate. I use three transects across the trench and accretionary prism to investigate differences in sediment consolidation and frontal wedge development due to seamounts on the incoming plate, and discuss potential implications for slow slip events


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