Stress-state differences between sedimentary cover and basement of the Songliao Basin, NE China: In-situ stress measurements at 6–7 km depth of an ICDP Scientific Drilling borehole (SK-II)

Abstract

The stress state down to the basement within sedimentary basins yet remains poorly understood. The anelastic strain recovery (ASR) method was employed to measure the stress state in the SK-II borehole in the Songliao Basin, Northeast China. The results show that the stress state differs significantly between the sedimentary cover and basement. In the sedimentary cover (6296–6335 m), the maximum principal stress σ1 is nearly vertical, which is in the normal faulting stress regime dominated by gravitation, in accordance with the normal faults observed on seismic reflection profiles within the sedimentary cover. In contrast, the basement (6646–6846 m) shows that σ1 is nearly horizontal, suggesting a strike-slip or reverse-faulting regime. The stress state of the basement is close to the reasonable stress condition for the Eurasian Plate far from a western Pacific plate subduction zone and consistent with the focal mechanisms of earthquakes (7–15 km) in the vicinity of ICDP SK-II borehole. The difference of stress state may imply that the far-field stress generated by the western Pacific plate has limited effects on the sedimentary cover, an increase in thickness correlates with an increasing influence of tectonic stress with depth.