T017-0013
Constraints on paleo-stress magnitude in Hikurangi Margin, New Zealand, at Site U1518, Expedition 375
Constraints on paleo-stress magnitude in Hikurangi Margin, New Zealand, at Site U1518, Expedition 375
Wednesday, 9 December 2020
Poster
Abstract:
Understanding stress and strain in subduction zones is essential to understand earthquake-slip behaviours.
Site U1518 of International Ocean Discovery Program (IODP) Expedition 375 is located at the toe of the Hikurangi subduction margin (North Island, New Zealand). In cores from this site, normal faults, fractures and a small number of thrust faults coexist. This is indicative of stress changes in the past. We conducted paleo-stress analysis to examine the stress changes in the shallow portion of Hikurangi margin, New Zealand.
The deformation structures are distributed mainly below around 300 m where two fractured fault zones were identified. Although the cross-cutting relationship is not clear based on the limited core-observations, the distributions of the structures overlap. Paleomagnetic declinations were used to reorient the faults into a geographic reference frame. Because open and filled fractures show no visible displacement, paleo-stress analysis was conducted using the dike method. Many normal faults have no striations, therefore, we used them as sense data only, using a micro-fault inversion method.
The estimated orientation of σ1is NW-SE or WNW-ESE and σ3 is vertical for open and filled fractures, which indicates thrust-type stress. The stress ratio, (σ1– σ2) / (σ1– σ3), is about 0.5. For the normal faults, σ1 is vertical and the stress ratio is 0.5.
Both thrust- and normal-fault stress states were observed in the cores at U1518 for the brittle faults or fractures, which suggests that both compressional and extensional critical states were achieved during the evolution of this system. Using stress polygons with the stress orientations and the stress ratio, the stress magnitudes were constrained for each stress state, allowing us to estimate variations in pore pressure ratio. The range for maximum shear stress is about 1-3 MPa. Additionally, applying dynamic Coulomb wedge theory (Wang and Hu, 2006) to the high angle slope at the toe of Hikurangi margin, the basal effective friction coefficient and shear stress around 300 m depth for each state can be calculated. Accordingly, the shear stress difference between the compressional and extensional critical state was about 1-2 MPa. The shear stress change determined from both methods (stress polygons and dynamic critical taper model) are mostly consistent.
Site U1518 of International Ocean Discovery Program (IODP) Expedition 375 is located at the toe of the Hikurangi subduction margin (North Island, New Zealand). In cores from this site, normal faults, fractures and a small number of thrust faults coexist. This is indicative of stress changes in the past. We conducted paleo-stress analysis to examine the stress changes in the shallow portion of Hikurangi margin, New Zealand.
The deformation structures are distributed mainly below around 300 m where two fractured fault zones were identified. Although the cross-cutting relationship is not clear based on the limited core-observations, the distributions of the structures overlap. Paleomagnetic declinations were used to reorient the faults into a geographic reference frame. Because open and filled fractures show no visible displacement, paleo-stress analysis was conducted using the dike method. Many normal faults have no striations, therefore, we used them as sense data only, using a micro-fault inversion method.
The estimated orientation of σ1is NW-SE or WNW-ESE and σ3 is vertical for open and filled fractures, which indicates thrust-type stress. The stress ratio, (σ1– σ2) / (σ1– σ3), is about 0.5. For the normal faults, σ1 is vertical and the stress ratio is 0.5.
Both thrust- and normal-fault stress states were observed in the cores at U1518 for the brittle faults or fractures, which suggests that both compressional and extensional critical states were achieved during the evolution of this system. Using stress polygons with the stress orientations and the stress ratio, the stress magnitudes were constrained for each stress state, allowing us to estimate variations in pore pressure ratio. The range for maximum shear stress is about 1-3 MPa. Additionally, applying dynamic Coulomb wedge theory (Wang and Hu, 2006) to the high angle slope at the toe of Hikurangi margin, the basal effective friction coefficient and shear stress around 300 m depth for each state can be calculated. Accordingly, the shear stress difference between the compressional and extensional critical state was about 1-2 MPa. The shear stress change determined from both methods (stress polygons and dynamic critical taper model) are mostly consistent.