Stress reversal recorded in calcite vein cuttings from the Nankai accretionary prism, southwest Japan

Abstract

The Nankai Trough subduction zone in southwest Japan is a typical convergent margin where the Philippine Sea plate subducts in the northwest direction beneath the Eurasian plate, and devastating earthquakes have repeatedly occurred in this region in the past. In order to investigate the evolution of the stress state in the subduction zone, we analyzed deformation microstructures and the preferred orientation of calcite grains in two cuttings of calcite veins from Hole C0002F that was drilled through the inner wedge of the Nankai accretionary prism during the Integrated Ocean Discovery Program (IODP) Expedition 338 in 2012. For both samples collected at depths of 1,085.5 and 1,885.5 meters below the sea floor (mbsf), the c-axes of calcite grains are preferentially oriented perpendicular to the vein wall, which is indicative of competitive growth of calcite during the vein opening caused by a vein normal extension. Also, mechanical e-twins were developed in both samples, and these are inferred to have been developed under the same stress field as that responsible for the formation of calcite veins based on the paleostress analyses in grains with e-twins. For the calcite vein retrieved at the depth of 1,885.5 mbsf, kink bands were also developed by the compression in the direction perpendicular to the vein wall, which is indicative of stress reversal after the formation of mechanical e-twins. Although we could not reach a definite conclusion for the cause of the stress reversal, it could have occurred during either fold development or seismic cycles in the Nankai accretionary prism.

In situ stress magnitude and rock strength in the Nankai accretionary complex: a novel approach using paired constraints from downhole data in two wells

“…The inconsistency between our result and the imaging of thrust faults in the accretionary wedge could indicate that these structures are no longer active (Boston et al 2016), that S Hmax and S hmin increase with depth to become the maximum and intermediate principal stresses, or that the stress regime changes through the earthquake cycle and will build toward a thrust regime as strain accumulates during the interseismic period (e.g., Wang and Hu 2006). The hypothesis that horizontal stress magnitudes fluctuate during the seismic cycle is broadly consistent with temporal variations in stress interpreted from analyses of calcite crystallography in samples from the accretionary prism (Takeshita et al 2014).…”

Site C0002

“…Orthogonal vein sets appear to be mutually crosscutting, suggesting that vein formation occurred as a result of temporal variations in the orientation of the stress field (e.g., Takeshita et al, 2014) possibly facilitated by elevated fluid pressure and small differential stress.…”

Microstructural analysis and calcite piezometry on hydrothermal veins: Insights into the deformation history of the Cocos Plate at Site U1414 (IODP Expedition 344)

“…Natural low strain deformation in calcite is documented by Takeshita et al . [] and McNamara et al . [] and experimental low strain deformation by Rybacki et al .…”