Stress state and patterns at the upper plate of Hikurangi Subduction Margin

Abstract

<p>Quantifying the contemporary stress state of the Earth’s crust is critical for developing a geomechanical understanding of the behavior of brittle deformation (fractures and faults).In this study we characterize the shallow contemporary stress state of the active Hikurangi Subduction Margin (HSM), New Zealand, to better understand how it affects and responds to variable deformation and slip behavior documented along this plate boundary. The HSM is characterized by along-strike variations in megathrust slip behavior, ranging from shallow slow slip events (SSEs) and creep at the northern and central HSM to interseismic locking and stress accumulation in the southern HSM. We estimate the state of stress across the HSM utilizing rock strength estimates from empirical relationships, leak-off test data, wireline logs and borehole geology, and measurement of borehole wall failures such as borehole breakouts and drilling‐induced tensile fractures from eight boreholes. Stress magnitude constraints at depth intervals where BOs are observed indicate that the maximum principal stress (σ<sub>1</sub>) is horizontal along the shallow (<3 km) HSM and the stress state is predominantly strike-slip or contractional (barring localized areas where an extensional stress state is determined). Our results reveal a NE-SW (margin-parallel) S<sub>Hmax</sub> orientation in the shallow central HSM, which rotates to a WNW- ESE/NW-SE (margin-perpendicular) S<sub>Hmax</sub> orientation in the shallow southern HSM. The central NE-SW S<sub>Hmax</sub> orientation is inconsistent with active, km-scale, NE-SW striking contractional faults observed across the central HSM. Considering both stress magnitude and orientation patterns at the central HSM, we suggest that long-term clockwise rotation of the Hikurangi forearc, over time, may transform motion on these km-scale central HSM faults from contractional dip-slip to a more contemporary strike/oblique-slip. The southern shallow WNW- ESE/NW-SE S<sub>Hmax</sub> orientation is nearly perpendicular to focal-mechanisms derived NE-SW S<sub>Hmax</sub> orientations within the subducting slab. This, combined with observed strike-slip and contractional faulting in the region and the NW-SE convergence direction, implies the overriding plate in the southern HSM is in a contractional stress state, potentially as deep as the plate interface, which is decoupled from that experienced in the subducting slab. Observed localized extensional stress states across the HSM may occur as a result of local extensions or reflect uncertainties in our estimations of S<sub>Hmax</sub> magnitude which are sensitive to the UCS values used (unconstrained by laboratory testing). This UCS uncertainty and the potential errors it can introduce into a stress model highlights the importance of developing robust empirical relationships for UCS in regions where stress is a critical geological consideration for hazard and resource management.</p>