TY - JOUR
T1 - Seafloor overthrusting causes ductile fault deformation and fault sealing along the Northern Hikurangi Margin
AU - Morgan, Julia K.
AU - Solomon, Evan A.
AU - Fagereng, Ake
AU - Savage, Heather M.
AU - Wang, Maomao
AU - Meneghini, Francesca
AU - Barnes, Philip M.
AU - Bell, Rebecca E.
AU - French, Melodie E.
AU - Bangs, Nathan L.
AU - Kitajima, Hiroko
AU - Saffer, Demian M.
AU - Wallace, Laura M.
N1 - Publisher Copyright:
© 2022 Elsevier B.V.
PY - 2022/9/1
Y1 - 2022/9/1
N2 - IODP Site U1518, drilled during IODP Expeditions 372 and 375, penetrated a large-offset (∼6 km) thrust, the Pāpaku fault, rising from a megathrust that hosts recurring slow slip events along the Hikurangi margin. Although drilling intersected the fault zone at only ∼300 m below the seafloor within porous silty mudstone, it exhibits intense tectonic ductile deformation, including finely banded mudstones contorted into decimeter-scale folds; elongate mudstone clasts with grain tail complexes; stacked and truncated silt beds in distorted mudstones; and soft sediment injections. Locally, these ductile features are overprinted by brittle deformation, including normal faults, fracture arrays, and breccias. The more consolidated hanging wall is dominated by brittle structures, whereas the footwall exhibits ductile and brittle deformation that decreases in intensity with depth. The intense tectonic ductile deformation and asymmetric distribution of structures across the fault zone at Site U1518 can be explained by seafloor overthrusting. The emplacement of the hanging wall upon the footwall flat overrode high-porosity, undeformed, and previously unburied sediments, localizing shear deformation within these weak sediments. In contrast, the overconsolidated hanging wall preferentially experienced brittle deformation during folding and displacement. Interstitial pore water geochemical profiles at Site U1518 show a repetition of near-seafloor diagenetic sequences below the fault, consistent with overthrusting of previously unburied strata. The preserved diagenetic profiles in the footwall suggest that overthrusting occurred within the last 50-100 kyr, and indicate little along- or across-fault fluid flow at the location of Site U1518. Thus the Pāpaku fault appears to define a low-permeability seal that restricts footwall consolidation, maintaining locally high pore fluid pressures and low fault strength. If similar low permeability structures occur elsewhere along the margin, they could support regionally high pore pressure conditions favorable to the occurrence of SSEs on the Hikurangi megathrust fault.
AB - IODP Site U1518, drilled during IODP Expeditions 372 and 375, penetrated a large-offset (∼6 km) thrust, the Pāpaku fault, rising from a megathrust that hosts recurring slow slip events along the Hikurangi margin. Although drilling intersected the fault zone at only ∼300 m below the seafloor within porous silty mudstone, it exhibits intense tectonic ductile deformation, including finely banded mudstones contorted into decimeter-scale folds; elongate mudstone clasts with grain tail complexes; stacked and truncated silt beds in distorted mudstones; and soft sediment injections. Locally, these ductile features are overprinted by brittle deformation, including normal faults, fracture arrays, and breccias. The more consolidated hanging wall is dominated by brittle structures, whereas the footwall exhibits ductile and brittle deformation that decreases in intensity with depth. The intense tectonic ductile deformation and asymmetric distribution of structures across the fault zone at Site U1518 can be explained by seafloor overthrusting. The emplacement of the hanging wall upon the footwall flat overrode high-porosity, undeformed, and previously unburied sediments, localizing shear deformation within these weak sediments. In contrast, the overconsolidated hanging wall preferentially experienced brittle deformation during folding and displacement. Interstitial pore water geochemical profiles at Site U1518 show a repetition of near-seafloor diagenetic sequences below the fault, consistent with overthrusting of previously unburied strata. The preserved diagenetic profiles in the footwall suggest that overthrusting occurred within the last 50-100 kyr, and indicate little along- or across-fault fluid flow at the location of Site U1518. Thus the Pāpaku fault appears to define a low-permeability seal that restricts footwall consolidation, maintaining locally high pore fluid pressures and low fault strength. If similar low permeability structures occur elsewhere along the margin, they could support regionally high pore pressure conditions favorable to the occurrence of SSEs on the Hikurangi megathrust fault.
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U2 - 10.1016/j.epsl.2022.117651
DO - 10.1016/j.epsl.2022.117651
M3 - Article
AN - SCOPUS:85132215514
SN - 0012-821X
VL - 593
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
M1 - 117651
ER -