Asymmetric brittle deformation at the Pāpaku Fault, Hikurangi Subduction Margin, NZ, IODP Expedition 375

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

Slow slip source characterized by lithological and geometric heterogeneity

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust

Expedition 372B/375 summary

Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expeditions 372 and 375 were undertaken to investigate the processes and in situ conditions that underlie subduction zone SSEs at the northern Hikurangi Trough. We accomplished this goal by (1) coring and geophysical logging at four sites, including penetration of an active thrust fault (the Pāpaku fault) near the deformation front, the upper plate above the SSE source region, and the incoming sedimentary succession in the Hikurangi Trough and atop the Tūranganui Knoll seamount; and (2) installing borehole observatories in the Pāpaku fault and in the upper plate overlying the slow slip source region. Logging-while-drilling (LWD) data for this project were acquired as part of Expedition 372, and coring, wireline logging, and observatory installations were conducted during Expedition 375. Northern Hikurangi subduction margin SSEs recur every 1–2 y and thus provide an ideal opportunity to monitor deformation and associated changes in chemical and physical properties throughout the slow slip cycle. In situ measurements and sampling of material from the sedimentary section and oceanic basement of the subducting plate reveal the rock properties, composition, lithology, and structural character of material that is transported downdip into the SSE source region. A recent seafloor geodetic experiment raises the possibility that SSEs at northern Hikurangi may propagate to the trench, indicating that the shallow thrust fault (the Pāpaku fault) targeted during Expeditions 372 and 375 may also lie in the SSE rupture area and host a portion of the slip in these events. Hence, sampling and logging at this location provides insights into the composition, physical properties, and architecture of a shallow fault that may host slow slip. Expeditions 372 and 375 were designed to address three fundamental scientific objectives: Characterize the state and composition of the incoming plate and shallow fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth and which may itself host shallow slow slip; Characterize material properties, thermal regime, and stress conditions in the upper plate directly above the SSE source region; and Install observatories in the Pāpaku fault near the deformation front and in the upper plate above the SSE source to measure temporal variations in deformation, temperature, and fluid flow. The observatories will monitor volumetric strain (via pore pressure as a proxy) and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface

Slow slip source characterized by lithological and geometric heterogeneity

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust

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

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

Trench floor depositional response to glacio-eustatic changes over the last 45 ka, northern Hikurangi subduction margin, New Zealand

Glacio-eustatic cycles lead to changes in sedimentation on all types of continental margins. There is, however, a paucity of sedimentation rate data over eustatic sea-level cycles in active subduction zones. During International Ocean Discovery Program Expedition 375, coring of the upper ∼110 m of the northern Hikurangi Trough Site U1520 recovered a turbidite-dominated succession deposited during the last ∼45 kyrs (Marine Isotope Stages (MIS) 1–3). We present an age model integrating radiocarbon dates, tephrochronology, and δ18O stratigraphy, to evaluate the bed recurrence interval (RI) and sediment accumulation rate (SAR). Our analyses indicate mean bed RI varies from ∼322 yrs in MIS1, ∼49 yrs in MIS2, and ∼231 yrs in MIS3. Large (6-fold) and abrupt variations in SAR are recorded across MIS transitions, with rates of up to ∼10 m/kyr occurring during the Last Glacial Maximum (LGM), and <1 m/kyr during MIS1 and 3. The pronounced variability in SAR, with extremely high rates during the LGM, even for a subduction zone, are the result of changes in regional sediment supply associated with climate-driven changes in terrestrial catchment erosion, and critical thresholds of eustatic sea-level change altering the degree of sediment bypassing the continental shelf and slope via submarine canyon systems

Compactive deformation of incoming calcareous pelagic sediments, northern Hikurangi subduction margin, New Zealand: Implications for subduction processes

Calcareous rocks are commonly found in subduction zones, but few studies have investigated the consolidation and compactive deformation of these rocks prior to subduction, and their potential effects on subduction and accretionary processes are thus poorly understood. Using drilling data obtained during International Ocean Discovery Program (IODP) Expeditions 372 and 375 combined with 2D and 3D seismic reflection data, the structure, growth history, and slip rates of normal faults identified in the incoming pelagic sedimentary sequences of the Hikurangi Margin were investigated. A seismic coherence depth slice and vertical profiles show that these faults exhibit polygonal structure that has rarely been documented at subduction margins. The polygonal faults are closely spaced and layer-bound within sequences dominated by pelagic carbonate and calcareous mudstone of Paleocene-Pliocene age. Kinematic modeling and 2D displacement analysis reveal that fault throws decrease toward the upper and lower tipline. In detail, two groups of throw profiles are defined by locations of displacement maxima, possibly reflecting lateral variations in physical properties. The polygonal fault system (PFS) likely formed by syneresis processes that involve diagenetically induced shear failure and volumetric contraction of the pelagic unit associated with fluid escape. Fault growth sequences reveal multiple, weakly correlated intervals of contemporaneous seafloor deformation and sedimentation and allow estimates of fault slip rates. We find evidence for a significant increase in typical slip rates from 0.5-3 m/Ma during pelagic sedimentation to >20 m/Ma following the onset of terrigenous sedimentation. These observations suggest that rapid loading of the pelagic sediments by the trench-wedge facies was associated with renewed and faster growth of the PFS. The PFS will eventually be transported into the base of the accretionary wedge, enhancing geometric roughness and heterogeneity of materials along the megathrust, and providing inherited zones of weakness. Selective fault reactivation may facilitate deformation and episodic vertical fluid migration in the lower wedge associated with microearthquakes, tremor, and slow slip events