Contrasted porosity between the hanging-wall and the footwall of the
active Pāpaku thrust at IODP Site U1518: insights on deformation and
erosion history and sediment compaction state evolution during accretion
at the northern Hikurangi margin deformation front
Abstract
Attempts to determine physical property across thrust faults at
subduction zones through drilling, logging and core sampling have been
limited and restricted to exhumed accretionary prisms or shallow parts
of active wedges. However, characterizing porosity evolution across the
sedimentary section entering subduction zones and accreted sediments is
crucial to understand deformation history at accretionary margins
through determination of sediment trajectories, quantification of
transported volumes of sediments and fluids with related mechanical
responses and understanding deformation processes in and around fault
zones. International Ocean Discovery Program Expeditions 372 and 375
drilled, logged and cored the entering basin (Site U1520) and active
Pāpaku thrust (Site U1518) few kilometers landward of the northern
Hikurangi margin deformation front where tsunami earthquakes and
recurrent slow slip events occur. Here, we examine physical properties
evolution across the Pāpaku thrust at Site U1518 including geophysical
logging data, pore size distribution obtained by combining Nuclear
Magnetic Resonance and Mercury Injection Capillary Pressure, and
interstitial porosity that is representative of sediment compaction
state, and compare with that of Site U1520. Interstitial porosity is
determined by correcting total connected porosity from clay-bound water
content based on cation exchange capacity. We evidence strong variations
of physical properties across the thrust fault, with lower porosity,
higher P-wave velocity and resistivity in the hanging-wall than in the
footwall. We suggest that the porosity pattern at the Pāpaku thrust
evidences differences in maximum burial depth with an overcompacted
hanging-wall that has been uplifted, thrusted and concomitantly eroded
above a nearly normally consolidated younger footwall.