Asymmetric Brittle Deformation at the Pāpaku Fault, Hikurangi Subduction Margin, NZ, IODP Expedition 375

Abstract

Quantifying fault damage zones provides a window into stress distribution and rheology around faults. International Ocean Discovery Program (IODP) Expeditions 372/375 drilled an active thrust splay fault within the Hikurangi subduction margin. The fault, which is hosted in Pleistocene clastic sediments, is surrounded by brittle fractures and faults as well as ductile deformation features. We find that fracture density in the damage zone enveloping the fault is asymmetric, with the hanging wall showing greater overall fracture density and at greater distances from the fault than the footwall. Furthermore, the peak in fracture density occurs within an area of mesoscale folding and localized slip in the hanging wall rather than adjacent to the main fault zone. We attribute the asymmetry in damage to disparate deformation histories between the hanging wall and footwall, greater ductile deformation within the footwall, and/or dynamic stress asymmetry around a propagating rupture. Damage asymmetry is common at shallow depths in subduction zones and influences the mechanical and hydrological properties of the fault, such as channelized fluid flow and fault stability. Finally, we demonstrate that subduction zone faults show similar damage‐displacement scaling as continental faults.

Evidence of Seismic Slip on a Large Splay Fault in the Hikurangi Subduction Zone

“…Drilling-induced deformation should only affect material closest to biscuit boundaries, which were avoided during sampling, while hydrothermal fluids at this location are <100°C (Barnes et al, 2010;Cook et al, 2020), which is below what would be required for biomarker reaction here. Furthermore, we do not see a correlation between heating anomalies and fracture density (Savage et al, 2021), which we may expect if fluids or methane COFFEY ET AL. U  where reaction was not observed, the red dashed line indicates the minimum amount of reaction required to produce a thermal-maturity signal above background.…”

Spatial Variations of Deformation along a Strike-Slip Fault: A Case Study of Xianshuihe Fault Zone, Southwest China

“…The power indices of 0.52 and 0.73 for normal faults and strike-slip faults, respectively, are both less than 1, indicating that the development width of fault damage zones slows down with the continuous increase in fault displacement. Previous studies have shown that the threshold for this low displacement is approximately 2 × 10 3 m [50]. When the cumulative displacement is below this threshold, the rate of increase in the width of fault damage zones becomes more severe with the increase in displacement.…”

“…where S i is the mean fracture frequency for each surveying site, D is the fracture frequency (m 3 ), x is the distance from the fault (km), a is a constant, and n is the decay parameter, which characterizes the degree of decay of the fracture frequency in the damage zones; a larger decay parameter indicates that the damage is attenuated more rapidly [50].…”

“…The statistical results indicate that the fracture frequency and rock strength on both walls of the XSHF hold an obvious relationship with the distance from the fault core (Figure 4). By observing the rock outcrops in the field, power-law function fitting, and comparing the rock properties in the damage zones with the rocks of the same type but undeformed (protolith zone) [50,51], we can determine the extents and boundaries of the different types of damage zones associated with the XSHF (Table 1). When the fracture frequency of the rock mass is less than or equal to 20-30 fractures per cubic meter, such rock mass would be considered outside of the actual fault zone.…”

“…By comparing the damage decay parameters collected during the field survey from the damage zones of the two walls of the XSHF with the data compiled by Savage and Brodsky (2011), we found that it is very similar to other strike-slip faults, i.e., it experienced the same amount of displacement (Figure 5b). In order to fully understand and further clarify the relationship between rock damage decay and the rate of fault displacement, we compared numerous published data [50,59] with the findings of this study. Subsequently, we found that the fault displacement has a significant impact on the damage decay in the rock mass associated with different types of faults.…”