Browsing by Author "Han, S."

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    Basal Accretion Along the South Central Chilean Margin and Its Relationship to Great Earthquakes
    (Wiley, 2020) Bangs, N.L.; Morgan, J.K.; Tréhu, A.M.; Contreras‐Reyes, E.; Arnulf, A.F.; Han, S.; Olsen, K.M.; Zhang, E.
    The south central Chilean margin regularly produces many of the world's largest earthquakes and tsunami, including the 2010 Mw 8.8 Maule and 1960 Mw 9.5 Valdivia events. In 2017, we acquired seismic reflection data along ~1,000 km of the margin using the R/V Langseth's 15 km long receiver array and 108.2 l (6,600 in3) seismic source to image structures associated with these ruptures. We focus on the Valdivia segment with the largest coseismic slip (~40 m). The outer 40 km of the forearc is an accretionary wedge constructed primarily of stacked sedimentary packages with irregular lengths and thicknesses and little along‐strike continuity. Forearc structures indicate that the accretionary wedge grows primarily through basal accretion of the downgoing trench fill. The décollement propagates along a weak boundary near the top of the trench fill but occasionally branches downward into the underthrust sediment along bedding horizons, peeling off slices that are underplated to the forearc. The shallow décollement level and the rarity of underplating events allow most of the trench sediment to subduct. As a result, only ~30% of the incoming sediment has been accreted since the Early Pliocene. This implies that, on average, ~1 km of sediment must subduct beyond the outer forearc, an inference that is supported by our seismic images. We propose that the thickness and great downdip and along‐strike extent of the underthrust layer, which separates the megathrust from the underlying roughness of the igneous ocean crust, ensures a smooth broad zone of strong coupling that generates the world's largest earthquakes and tsunami.
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    Nature and Origin of Magnetic Lineations Within Valdivia Bank: Ocean Plateau Formation by Complex Seafloor Spreading
    (Wiley, 2023) Thoram, S.; Sager, W. W.; Gaastra, K.; Tikoo, S. M.; Carvallo, C.; Avery, A.; Del Gaudio, Arianna V.; Huang, Y.; Hoernle, K.; Höfig, T. W.; Bhutani, R.; Buchs, D. M.; Class, C.; Dai, Y.; Valle, G. Dalla; Fielding, S.; Han, S.; Heaton, D. E.; Homrighausen, S.; Kubota, Y.; Li, C.-F.; Nelson, W. R.; Petrou, E.; Potter, K. E.; Pujatti, S.; Scholpp, J.; Shervais, J. W.; Tshiningayamwe, M.; Wang, X. J.; Widdowson, M.
    Valdivia Bank (VB) is a Late Cretaceous oceanic plateau formed by volcanism from the Tristan-Gough hotspot at the Mid-Atlantic Ridge (MAR). To better understand its origin and evolution, magnetic data were used to generate a magnetic anomaly grid, which was inverted to determine crustal magnetization. The magnetization model reveals quasi-linear polarity zones crossing the plateau and following expected MAR paleo-locations, implying formation by seafloor spreading over ∼4 Myr during the formation of anomalies C34n-C33r. Paleomagnetism and biostratigraphy data from International Ocean Discovery Program Expedition 391 confirm the magnetic interpretation. Anomaly C33r is split into two negative bands, likely by a westward ridge jump. One of these negative anomalies coincides with deep rift valleys, indicating their age and mechanism of formation. These findings imply that VB originated by seafloor spreading-type volcanism during a plate reorganization, not from a vertical stack of lava flows as expected for a large volcano.