Direct constraints on in situ stress state from deep drilling into the Nankai subduction zone, Japan

Abstract

Stress state is a long-sought but poorly known parameter on subduction megathrusts and in overlying accretionary wedges in general. We used direct observations made during drilling of Integrated Ocean Drilling Program (IODP) borehole C0002 to a depth of 3058 m below the seafloor (mbsf) in the Nankai subduction zone of southwestern Japan to constrain in situ pore pressure and stress state in the deep interior of an accretionary wedge for the first time. These data included downhole pressure, active pumping tests, and logging and sample measurements. We found a nearly linear gradient in minimum horizontal principal stress (Shmin) and show that it remained consistently smaller than the vertical stress (Sv), definitively ruling out a thrust-faulting stress regime to at least 3 km depth, and to within ~2 km above the subduction megathrust. At 3000 mbsf, the estimated effective stresses were: Sv = 33 MPa, SHmax = 25–36 MPa, and Shmin = 18.5–21 MPa. We therefore interpret that the stress state throughout the drilled interval, which lies entirely in the hanging wall of the active megathrust, lies in a normal or strike-slip faulting regime (Sv ≥ SHmax > Shmin). Total differential stresses are below ~18 MPa. We conclude that (1) basal traction along the megathrust must be small in order to permit both locking (and frictional sliding at failure) of the décollement and such low differential stresses deep within the upper plate; and (2) although differential stresses may remain low all the way to the plate boundary at ~5000 mbsf, SHmax must transition to become greater than the vertical stress—either spatially below the base of the borehole or temporally leading up to megathrust fault rupture—in order to drive thrust motion along the plate boundary as observed in great earthquakes and in recurring very low-frequency earthquakes and slow-slip events.

Accretion Cycles, Structural Evolution, and Thrust Activity in Accretionary Wedges With Various Décollement Configurations: Insights From Sandbox Analog Modeling

“…(c) Accretionary wedge deformed by subduction of a seamount at the Hikurangi margin (Barnes et al, 2002(Barnes et al, , 2010Nicol et al, 2007;Watson et al, 2019). (d) Accretionary wedge segmented by megasplay faults, which is considered to be a double décollement accretionary wedge at the Nankai margin (G. F. Moore et al, 2014;Tobin et al, 2022). Note that the colors are used to aid visibility, and that areas of the same color in the different accretionary wedges do not represent the same lithology.…”

“…(d) Accretionary wedge segmented by megasplay faults, being considered a doubledécollement accretionary wedge at the Nankai margin (G. F. Moore et al, 2014;Tobin et al, 2022). Note that the colors are used to aid visibility, and that areas of the same color in the different accretionary wedges do not represent the same lithology.…”