In-situ stress field and mechanics of fault reactivation in the Gediz Graben, Western Turkey

摘要

The present-day stress field in the Gediz Graben is characterized by vertical maximum and horizontal 105°N-trending intermediate and 015°N-trending minimum principal stress axes. Stress gradients are 23.4,18.6 and 15.3 MPa/km for maximum, intermediate and minimum stresses, respectively. This stress tensor aligns well with the observed fault pattern of the graben and confirms the ～N-S-oriented extension. The strikedip of optimum planes with highest slip tendency is 105°60°S and 285°60°N and highest dilation tendency is on 105°N-trending vertical fractures. Faults in the graben fill have near normal frictional strength with friction coefficient of (μ) 0.48. Pore pressure is generally hydrostatic but local overpressure elevated to pore-fluid factor (λ) of 0.71 was observed in close proximity to the master bounding fault of the graben. Data and geological evidence suggest that this fault is a major conduit for mantle degassing and plays a significant role in fluid circulation. The hot CO_2-rich fluids ascending through the fault system could be locally trapped to form overpressured CO_2 pockets as observed in the shale-rich Alasehir Formation which is suitable to form clay smears and clay-matrix fault gouges with high seal potential. High temperature and CO_2 content is also favorable for healing and sealing of the fractures by carbonate precipitation to re-establish fault cohesion. Temporary fault seal breaching may occur due to shear reactivation of the faults and could relieve excess pore pressure. This may also lead to surface discharge of CO_2-rich fluids with hydrocarbons generated in the Alajehir Formation. Projection of the deduced stress field to 6 km depth suggests that brittle reactivation of the low-angle segment of the master bounding fault possibly requires combined operation of overpressuring and fault zone weakening.