Rheological strength of the middle to upper crust in the Bohai Bay Basin and its implications for earthquake nucleation: Constraints from high-temperature, high-pressure experiments

The depth of the crustal brittle-plastic transition zone controls the vertical distribution of seismicity. However, in ancient cratons such as the North China craton that have experienced activation and thinning, the rheological properties of the middle crust in key tectonic units like the Bohai Bay Basin lack direct experimental constraints. To address this, we conducted deformation experiments on Archaean granitic gneiss recovered from borehole cores in the Bozhong Sag of the Bohai Bay Basin, using a Griggs solid-medium apparatus under temperatures of 500–900 °C and pressures of 300–500 MPa. Macroscopic and microstructural observations revealed brittle failure at 500 °C and 500 MPa and at 800 °C and 300 MPa. Integrated mechanical and microstructural analyses indicate that the gneiss deformed predominantly by semibrittle flow (stress exponent n = ~6.3, activation energy Q = ~144 kJ/mol) at 500 MPa and 600–700 °C, transitioning to dislocation creep–dominated plastic flow (n = ~4.6, Q = ~400 kJ/mol) at 500 MPa and 800–900 °C. The rheological strength of the gneiss is comparable to that of anorthite, suggesting that feldspar-rich lithologies control the strength of the Bohai Bay middle crust. Based on the high-temperature rheology and the regional geothermal gradient (3.5 °C/100 m), we predict the brittle-plastic transition depth to be ~12.2 km—shallower than that predicted by the North China crustal model and consistent with local seismicity patterns. This study provides the first experimental constraints on the middle-crustal rheology of the Bohai Bay Basin, demonstrating that a high geothermal gradient weakens the feldspar-dominated middle crust, uplifts the brittle-plastic transition zone, and thins the seismogenic layer. These results offer rheological evidence for understanding crustal strength, deformation mechanisms, and earthquake nucleation in the middle to upper crust of the basin.