In situ stress conditions at IODP Site C0002 reflecting the tectonic evolution of the sedimentary system near the seaward edge of the Kumano basin, offshore from SW Japan

Abstract

This paper presents a complete set of in situ stress calculations for depths of 200–1400 meters below seafloor at Integrated Ocean Drilling Program (IODP) Site C0002, near the seaward margin of the Kumano fore‐arc basin, offshore from southwest Japan. The vertical stress component was obtained by integrating bulk density calculations from moisture and density logging data, and the two horizontal components were stochastically optimized by minimizing misfits between a probabilistic model and measured breakout widths for every 30 m vertical segment of the wellbore. Our stochastic optimization process reveals that the in situ stress regime is decoupled across an unconformity between an accretionary complex and the overlying Kumano fore‐arc basin. The stress condition above the unconformity is close to the critical condition for normal faulting, while below the unconformity the geologic system is stable in a normal to strike‐slip fault stress regime. The critical state of stress demonstrates that the tectonic evolution of the sedimentary system has been achieved mainly by the regionally continuous action of a major out‐of‐sequence thrust fault during sedimentation in the fore‐arc basin. The stable stress condition in the accretionary prism is interpreted to have resulted from mechanical decoupling by the accommodation of large displacement along the megasplay fault.

In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002

“…Our analysis suggests that at Site C0002, S H is smaller than S v (normal faulting regime) in the Kumano Basin and becomes close to or greater than S v within the prism. Our estimation of stresses is generally consistent with borehole breakout analyses above 2,000 mbsf at Site C0002 (Chang et al, ; Chang & Song, ; Huffman et al, ; Song & Chang, ), although our estimated S H magnitudes below 1,350 mbsf are somewhat greater than those from the breakout analyses. One potential explanation for this difference is that as mentioned above, the assumption of an in situ CSL condition yields upper bound estimates of both S H and P p .…”

Strength Profile of the Inner Nankai Accretionary Prism at IODP Site C0002

“…The C 0 value of~7.1 MPa estimated from a series of triaxial deformation tests on cores from~2,200 mbsf (Kuo et al, 2017) is also within the range of our indentation test results. However, C 0 values constrained from borehole analysis (Huffman et al, 2016), and those estimated from the empirical C 0 -V p relationship (Olcott & Saffer, 2013;Song & Chang, 2017) above 1,400 mbsf are clearly higher by 5-13 MPa than the trend of C 0 values determined from our indentation tests. The empirical C 0 -V p relationships used in Song and Chang (2017) result in higher C 0 values than those determined in our indentation experiments ( Figure S4a).…”

“…However, C 0 values constrained from borehole analysis (Huffman et al, 2016), and those estimated from the empirical C 0 -V p relationship (Olcott & Saffer, 2013;Song & Chang, 2017) above 1,400 mbsf are clearly higher by 5-13 MPa than the trend of C 0 values determined from our indentation tests. The empirical C 0 -V p relationships used in Song and Chang (2017) result in higher C 0 values than those determined in our indentation experiments ( Figure S4a). A range of C 0 determined in Huffman et al (2016) is mostly higher than our estimated C 0 .…”

“…C i decreases with depth from~27 MPa at 2,400 mbsf to~12 MPa at~2,800 mbsf, followed by a slight increase to~22 MPa at~3,000 mbsf. Although the (Kitajima et al, 2012;Kuo et al, 2017; the data provided by CDEX), the C 0 values estimated from the breakout analysis (Huffman et al, 2016) and the C 0 values derived from the empirical relation between the V p and C 0 (Olcott & Saffer, 2013;Song & Chang, 2017). Errors of C i and C 0 determined in this study are mostly smaller than the symbols.…”

“…Depth profiles at Site C0002 of (a) indentation strength C i , (b) unconfined compressive strength C 0 , (c) in situ rock strength in the normal faulting regime (NF) where S 1 = S v , and (d) in situ rock strength in the reverse faulting regime (RF) where S 3 = S v . Also shown in (b) are C 0 determined from triaxial compression experiments on cores (Kitajima et al, ; Kuo et al, ; the data provided by CDEX), the C 0 values estimated from the breakout analysis (Huffman et al, ) and the C 0 values derived from the empirical relation between the V p and C 0 (Olcott & Saffer, ; Song & Chang, ). Errors of C i and C 0 determined in this study are mostly smaller than the symbols.…”